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Institute for Theoretical Chemistry (T3.0)
Research
A. Köhn
Publications

Publications

Publications by the Köhn Group

Publications of the Köhn Group

2024
1. Suhr, S., Hunger, D., Walter, R. R. M., Köhn, A., Slageren, J. van, & Sarkar, B. “Air-Stable Dinuclear Complexes of Four-Coordinate ZnII and NiII Ions with a Radical Bridge: A Detailed Look at Redox Activity and Antiferromagnetic Coupling.” Inorg. Chem. 63, 6042–6050 (2024)
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  @article{Suhr_Hunger_Walter_Köhn_Slageren_Sarkar_2024, abstractnote = {Air-stable dinuclear complexes [(bmsab)NiII(tmsab)NiII(bmsab)]3– and [(bmsab)ZnII(tmsab)ZnII(bmsab)]3– (bmsab = bis(methanesulfoneamido)benzene, tmsab = tetra(methanesulfonamido)benzene) were prepared via a synthetic route based on heteroleptic precursor complexes. The new complexes combine a distorted tetrahedral coordination environment with an open-shell bridging ligand. The ZnII species was subjected to a detailed investigation of the (spectro-)electrochemical processes. The NiII species is a rare example of a complex that combines strong exchange coupling (J > 440 cm–1) with pronounced positive zero-field splitting (D = +72 cm–1). Combining SQUID magnetometry and (HF)EPR spectroscopy with ab initio calculations allowed for accurate quantification of the exchange interaction.}, author = {Suhr, Simon and Hunger, David and Walter, Robert R. M. and Köhn, Andreas and Slageren, Joris van and Sarkar, Biprajit}, doi = {10.1021/acs.inorgchem.4c00351}, issn = {0020-1669}, journal = {Inorg. Chem.}, number = 13, pages = {6042–6050}, title = {Air-Stable Dinuclear Complexes of Four-Coordinate ZnII and NiII Ions with a Radical Bridge: A Detailed Look at Redox Activity and Antiferromagnetic Coupling}, url = {https://doi.org/10.1021/acs.inorgchem.4c00351}, volume = 63, year = 2024 }
  Link
  https://doi.org/10.1021/acs.inorgchem.4c00351
2. Rau, T., Sedlmair, M., & Köhn, A. “chARpack: The Chemistry Augmented Reality Package.” Journal of Chemical Information and Modeling 64, 4700–4708 (2024)
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  @article{Rau_Sedlmair_Köhn_2024, abstractnote = {Off-loading visualization and interaction into virtual reality (VR) using head-mounted displays (HMDs) has gained considerable popularity in simulation sciences, particularly in chemical modeling. Because of its unique way of soft immersion, augmented reality (AR) HMD technology has even more potential to be integrated into the everyday workflow of computational chemists. In this work, we present our environment to explore the prospects of AR in chemistry and general molecular sciences: The chemistry in Augmented Reality package (chARpack). Besides providing an extensible framework, our software focuses on a seamless transition between a 3D stereoscopic view with true 3D interactions and the traditional desktop PC setup to provide users with the best setup for all tasks in their workflow. Using feedback from domain experts, we discuss our design requirements for this kind of hybrid working environment (AR + PC), regarding input, features, degree of immersion, and collaboration.}, author = {Rau, Tobias and Sedlmair, Michael and Köhn, Andreas}, doi = {10.1021/acs.jcim.4c00462}, issn = {1549-9596}, journal = {Journal of Chemical Information and Modeling}, number = 12, pages = {4700–4708}, title = {chARpack: The Chemistry Augmented Reality Package}, volume = 64, year = 2024 }
3. Zielinski, P., & Köhn, A. “Coupled-Cluster Theories for Excited States.” In M. Yanez & R. J. Boyd (Eds.), Excited States and Photodynamic Simulations from Photobiology to Photomaterials (pp. 116–140) (2024)
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  @inbook{Zielinski_Köhn_2024, abstractnote = {Coupled-cluster theory offers a hierarchy of increasingly accurate methods and provides thus an important basis for accurate quantum chemistry, also for the computation of electronic excited states. This chapter explains and compares the two main approaches, equation-of-motion and linear-response theory and sketches the computation of transition moments and expectation values, as well as analytic geometric gradients. The basic approaches to arrive at approximations are discussed, and recent benchmark works are used to demonstrate their relative accuracy. Some challenges in coupled-cluster theory, like going to large systems, open-shell and multireference theory and the slow basis-set convergence are also covered.}, author = {Zielinski, Patrik and Köhn, Andreas}, doi = {10.1016/b978-0-12-821978-2.00035-0}, editor = {Yanez, Manuel and Boyd, Russell J.}, journal = {Excited States and Photodynamic Simulations from Photobiology to Photomaterials}, pages = {116–140}, title = {Coupled-Cluster Theories for Excited States}, url = {https://doi.org/10.1016/b978-0-12-821978-2.00035-0}, year = 2024 }
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  https://doi.org/10.1016/b978-0-12-821978-2.00035-0
4. Klostermann, S. V., Kappler, J., Waigum, A., Buchmeiser, M. R., Köhn, A., & Kästner, J. “The reduction behavior of sulfurized polyacrylonitrile (SPAN) in lithium–sulfur batteries using a carbonate electrolyte: a computational study.” Phys. Chem. Chem. Phys. 26, 9998–10007 (2024)
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  @article{klo24, author = {Klostermann, S. V. and Kappler, J. and Waigum, A. and Buchmeiser, M. R. and K\"ohn, A. and K\"astner, J.}, doi = {10.1039/D3CP06248A}, journal = {Phys. Chem. Chem. Phys.}, pages = {9998-10007}, title = {The reduction behavior of sulfurized polyacrylonitrile ({SPAN}) in lithium–sulfur batteries using a carbonate electrolyte: a computational study}, volume = 26, year = 2024 }
2023
1. Zielinski, P., Black, J. A., & Köhn, A. “Performance Tests of the Second-Order Approximate Internally Contracted Multireference Coupled-Cluster Singles and Doubles Method icMRCC2.” J. Chem. Theory Comput. 19, 8671–8688 (2023)
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  @article{zielinski2023performance, abstractnote = {Benchmark results are presented for the second-order approximation of the internally contracted multireference coupled-cluster method with single and double excitations, icMRCC2 [Köhn, Bargholz, J. Chem. Phys. 2019, 151, 041106], which was designed as a multireference analogue of the single-reference second-order approximate coupled-cluster method CC2 [Christiansen, Koch, Jørgensen, Chem. Phys. Lett. 1995, 243, 409–418]. Vertical excitation energies of various small to medium-sized organic molecules are investigated based on established test sets from the literature. Additionally, the spectroscopic constants of ground and excited states of diatomics and the geometric parameters of excited triatomic molecules were determined and compared to the experimental data. The results show that the method clearly extends the applicability of single-reference CC2, including doubly excited states, and also artifacts of CC2 like too low Rydberg excitations and too weak multiple bonds are eliminated. The method is computationally more demanding than standard multireference second-order perturbation theories but improves significantly in accuracy, as shown by the benchmark results. In addition, it is demonstrated that small active spaces are often sufficient to obtain accurate energies with icMRCC2. Example applications like the automerization of cyclobutadiene, the deactivation pathway of ethylene, and the excited states of an iron complex with a noninnocent nitrosyl ligand demonstrate the potential of icMRCC2 in cases with strong multireference character.}, author = {Zielinski, Patrik and Black, Joshua A. and Köhn, Andreas}, doi = {10.1021/acs.jctc.3c00969}, issn = {1549-9618}, journal = {J. Chem. Theory Comput.}, number = 23, pages = {8671–8688}, title = {Performance Tests of the Second-Order Approximate Internally Contracted Multireference Coupled-Cluster Singles and Doubles Method icMRCC2}, url = {https://doi.org/10.1021/acs.jctc.3c00969}, volume = 19, year = 2023 }
  Link
  https://doi.org/10.1021/acs.jctc.3c00969
2. Black, J. A., Waigum, A., Adam, R. G., Shamasundar, K. R., & Köhn, A. “Towards an efficient implementation of internally contracted coupled-cluster methods.” J. Chem. Phys. 158, 134801 (2023)
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  @article{Black_Waigum_Adam_Shamasundar_Köhn_2023, abstractnote = {A new implementation of the internally contracted multireference coupled-cluster with singles and doubles (icMRCCSD) method is presented. The new code employs an efficient tensor contraction kernel and can also avoid full four-external integral transformations, which significantly extends the scope of the applicability of icMRCCSD. The new implementation is currently restricted to the simple case of two active electrons in two orbitals and also supports the computation of spin-adapted doublet and triplet coupled-cluster wavefunctions. This contribution describes the basic approach for the automated derivation of the working equations and benchmarks the current code against efficient implementations of standard methods such as single-reference coupled-cluster singles and doubles (CCSD) and internally contracted multireference configuration interaction (icMRCI). The run times for linearized variants of icMRCCSD are only twice as long as comparable CCSD runs and similar to those of the icMRCI implementation, while the non-linear terms of more complete variants of icMRCCSD lead to an order of magnitude longer computation times. Nevertheless the new code allows for computations at larger scales than it was possible previously, with less demands on memory and disk-space resources. This is exemplified by numerical structure optimizations and harmonic force field determinations of NC 2 H 5 isomers and the singlet and triplet state of m-benzyne. In addition, the exchange coupling of a dinuclear copper complex is determined. This work also defines a new commutator approximation for icMRCCSD which includes all terms that are also present in the single-reference CCSD method, thus yielding a consistent pair of single-reference and multireference coupled-cluster methods.}, author = {Black, Joshua Alexander and Waigum, Alexander and Adam, Robert G and Shamasundar, K R and Köhn, Andreas}, doi = {10.1063/5.0143214}, issn = {0021-9606}, journal = {J. Chem. Phys.}, pages = 134801, title = {Towards an efficient implementation of internally contracted coupled-cluster methods}, volume = 158, year = 2023 }
2022
1. Ma, S., Liu, W., & Köhn, A. “Adsorption Geometries and Electronic Properties of a Dipolar Phosphonate-Based Monolayer on the NiO Surface.” J. Phys. Chem. C 126, 5793–5804 (2022)
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  @article{Ma_Liu_Köhn_2022, abstractnote = {Phosphonates have been verified experimentally to have substantial influence on the performance of solution-processed nickel oxide (sNiO)-based organic electronic devices. However, a fully atomistic understanding of the phosphonate/sNiO interface is still lacking. Therefore, based on first-principles calculations, the interface of 4-cyanophenylphosphonic acid (CYNOPPA) molecules and the sNiO surface was studied comprehensively to clarify the grafting process. sNiO was modeled by a variety of reconstructed NiO(111) surfaces with different amounts of hydroxylation, as well as by NiO(100) and β-Ni(OH)2(001) surfaces. We discuss the adsorption geometries and energies of CYNOPPA on these surfaces, as well as the evolution of the microstructures due to thermal energy, and show the impact on the work function and thermodynamic driving forces for hole transport. The results indicate that CYNOPPA molecules adsorb on the sNiO surface in a variety of binding modes. Independent of the binding mode, the adsorption process always leads to a positive charging of the sNiO surface, with the counter charges in the adsorbed CYNOPPA molecules, either by direct proton transfer or by H2O elimination from the interface during the adsorption process. Consequently, the work function of the sNiO surface increases upon CYNOPPA adsorption, partially enhanced by the inherent dipole moment of CYNOPPA. The highest occupied molecular orbital of CYNOPPA is always below the valence band maximum and thus facilitates hole injection.}, author = {Ma, Shuangying and Liu, Wenlan and Köhn, Andreas}, doi = {10.1021/acs.jpcc.2c01308}, issn = {1932-7447}, journal = {J. Phys. Chem. C}, number = 12, pages = {5793–5804}, title = {Adsorption Geometries and Electronic Properties of a Dipolar Phosphonate-Based Monolayer on the NiO Surface}, volume = 126, year = 2022 }
2. Köhn, A., & Olsen, J. “Capabilities and limits of the unitary coupled-cluster approach with generalized two-body cluster operators.” J. Chem. Phys. 157, 124110 (2022)
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  @article{Koehn2022gucc, abstract = {Unitary cluster expansions of the electronic wavefunction have recently gained much interest because of their use in conjunction with quantum algorithms. In this contribution, we investigate some aspects of an ansatz, using generalized two-body excitation operators, which have been considered in some recent studies on quantum algorithms for quantum chemistry. Our numerical results show that, in particular, two-body operators with effective particle–hole excitation level of one in connection with the usual particle–hole double excitation operators lead to a very accurate, yet compact representation of the wavefunction. Generalized two-body operators with effective excitation rank zero have a considerably less pronounced effect. We compare with standard and unitary coupled-cluster expansions and show that the above mentioned approach matches or even surpasses the accuracy of expansions with three-body particle–hole excitations, in particular at the onset of strong correlation. A downside of the approach is that it is rather difficult to rigorously converge it to its variational minimum.}, author = {Köhn, Andreas and Olsen, Jeppe}, doi = {10.1063/5.0104815}, issn = {0021-9606}, journal = {J. Chem. Phys.}, number = 12, pages = 124110, title = {Capabilities and limits of the unitary coupled-cluster approach with generalized two-body cluster operators}, url = {https://dx.doi.org/10.1063/5.0104815}, volume = 157, year = 2022 }
  Link
  https://dx.doi.org/10.1063/5.0104815
3. Stöckl, Y., Fellmeth, T., Bauer, F., Wank, B., Frey, W., Claasen, B., Zens, A., Köhn, A., & Laschat, S. “Chasing Polycyclic Natural Products: 5/6/5- or 5/6/6-Carbotricyclic Scaffold Construction via Stereodivergent Diels-Alder Reaction of Chiral Hydrindanes and Their Boron Complexes.” Eur. J. Org. Chem. 2022, e202101416 (2022)
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  @article{stockl2022chasing, abstractnote = {Chiral trans-hydrindanes (bicyclo[4.3.0]nonanes) are important building blocks of polycyclic natural products. In order to access 5/6/5- and 5/6/6-carbotricyclic scaffolds scope and limitation of [4+2] cycloadditions of tetrahydroindanones with various dienes were studied. Cyclopentadiene gave a tetracylic endo-(R,R)-diastereomer under acid-catalysis, whereas thermal conditions provided the endo-(S,S)-diastereomer with the opposite diastereofacial selectivity. The stereodivergent outcome was rationalized by high-level quantum-chemical computations which revealed the acid-catalysis to be a kinetically controlled reaction and the thermal cycloaddition to be under thermodynamic control. Stereochemical assignment of the cycloadducts was facilitated by conversion of the 1,3-dicarbonyls with BF3 ⋅ OEt2 into BF2-chelate complexes. Subsequent thermal Diels-Alder reaction of BF2- or BBN-chelates (from 9-BBN-OTf) gave endo/exo-mixtures of the (R,R)- and (S,S)-diastereomers, while more elevated temperatures yielded primarily the endo/exo-(S,S)-diastereomers. Thermal [4+2] cycloadditions with 2,3-dimethylbutadiene proceeded with lower diastereoselectivity as the reaction was kinetically controlled according to calculations. Attempted Diels-Alder-reactions with furan gave furyl-substituted indanones rather than cycloadducts. The Diels-Alder reaction of 4-substituted tetrahydroindanones with cyclic and acyclic dienes was examined. By choosing acidic or thermal conditions, the diastereoselectivity on the newly formed ring could be controlled and was rationalized by high-level computations. Additionally, boron complexation of the 1,3-dicarbonyl moiety exhibited unexpected selectivity in the cycloaddition.}, author = {Stöckl, Yannick and Fellmeth, Thomas and Bauer, Florian and Wank, Bianca and Frey, Wolfgang and Claasen, Birgit and Zens, Anna and Köhn, Andreas and Laschat, Sabine}, doi = {10.1002/ejoc.202101416}, issn = {1434-193X}, journal = {Eur. J. Org. Chem.}, number = 26, pages = {e202101416}, title = {Chasing Polycyclic Natural Products: 5/6/5- or 5/6/6-Carbotricyclic Scaffold Construction via Stereodivergent Diels-Alder Reaction of Chiral Hydrindanes and Their Boron Complexes}, url = {https://dx.doi.org/10.1002/ejoc.202101416}, volume = 2022, year = 2022 }
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  https://dx.doi.org/10.1002/ejoc.202101416
4. Schnabel, J., Cheng, L., & Köhn, A. “High-accuracy Rb2+ interaction potentials based on coupled-cluster calculations.” Phys. Rev. A 106, 032804 (2022)
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  @article{Schnabel2022highAccu, abstractnote = {This work discusses a protocol for constructing highly accurate potential energy curves (PECs) for the lowest two states of Rb2+, i.e., X2Σg+ and (1)2Σu+, using an additivity scheme based on coupled-cluster theory. The approach exploits the findings of our previous work [J. Schnabel, L. Cheng, and A. Köhn, J. Chem. Phys. 155, 124101 (2021)10.1063/5.0062098] to avoid the unphysical repulsive long-range barrier occurring for symmetric molecular ions when perturbative estimates of higher-order cluster operators are employed. Furthermore, care was taken to reproduce the physically correct exchange splitting of the X2Σg+ and (1)2Σu+ PECs. The accuracy of our computational approach is benchmarked for ionization energies of Rb and for spectroscopic constants as well as vibrational levels of the a3Σu+ triplet state of Rb2 . We study high-level correlation contributions, high-level relativistic effects, and inner-shell correlation contributions and find very good agreement with experimental reference values for the atomic ionization potential and the binding energy of Rb2 in the a3Σu+ triplet state. Our final best estimate for the binding energy of the Rb2+ X2Σg+ state including zero-point vibrational contributions is D0=6179cm−1 with an estimated error bound of O(±30cm−1). This value is smaller than the experimentally inferred lower bond of D0≥6307.5cm−1 [M. A. Bellos et al., Phys. Rev. A 87, 012508 (2013)10.1103/PhysRevA.87.012508] and will require further investigation. For the (1)2Σu+ state a shallow potential with D0=78.4cm−1 and an error bound of ±9cm−1 is computed.}, author = {Schnabel, Jan and Cheng, Lan and Köhn, Andreas}, doi = {10.1103/physreva.106.032804}, issn = {2469-9926}, journal = {Phys. Rev. A}, number = 3, pages = 032804, title = {High-accuracy Rb2+ interaction potentials based on coupled-cluster calculations}, url = {https://dx.doi.org/10.1103/physreva.106.032804}, volume = 106, year = 2022 }
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  https://dx.doi.org/10.1103/physreva.106.032804
5. Gebhardt, P., Yu, X., Köhn, A., & Sedlmair, M. MolecuSense: Using Force-Feedback Gloves for Creating and Interacting with Ball-and-Stick Molecules in VR. Proceedings of the 15th International Symposium on Visual Information Communication and Interaction (pp. 1–5). Publication, New York, NY, USA: Association for Computing Machinery (2022)
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  @proceedings{gebhardt2022molecusense, abstract = {We contribute MolecuSense, a virtual version of a physical molecule construction kit, based on visualization in Virtual Reality (VR) and interaction with force-feedback gloves. Targeting at chemistry education, our goal is to make virtual molecule structures more tangible. Results of an initial user study indicate that the VR molecular construction kit was positively received. Compared to a physical construction kit, the VR molecular construction kit is on the same level in terms of natural interaction. Besides, it fosters the typical digital advantages though, such as saving, exporting, and sharing of molecules. Feedback from the study participants has also revealed potential future avenues for tangible molecule visualizations.}, address = {New York, NY, USA}, author = {Gebhardt, Patrick and Yu, Xingyao and Köhn, Andreas and Sedlmair, Michael}, booktitle = {Proceedings of the 15th International Symposium on Visual Information Communication and Interaction}, doi = {10.1145/3554944.3554956}, journal = {Proceedings of the 15th International Symposium on Visual Information Communication and Interaction}, pages = {1–5}, publisher = {Association for Computing Machinery}, series = {VINCI '22}, title = {MolecuSense: Using Force-Feedback Gloves for Creating and Interacting with Ball-and-Stick Molecules in VR}, type = {Publication}, url = {https://dx.doi.org/10.1145/3554944.3554956}, year = 2022 }
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  https://dx.doi.org/10.1145/3554944.3554956
6. Zaverkin, V., Netz, J., Zills, F., Köhn, A., & Kästner, J. “Thermally Averaged Magnetic Anisotropy Tensors via Machine Learning Based on Gaussian Moments.” J. Chem. Theory Comput. 18, 1–12 (2022)
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  @article{zav22, author = {Zaverkin, Viktor and Netz, Julia and Zills, Fabian and Köhn, Andreas and K\"astner, Johannes}, doi = {10.1021/acs.jctc.1c00853}, journal = {J. Chem. Theory Comput.}, pages = {1-12}, title = {Thermally Averaged Magnetic Anisotropy Tensors via Machine Learning Based on Gaussian Moments}, url = {http://dx.doi.org/10.1021/acs.jctc.1c00853}, volume = 18, year = 2022 }
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  http://dx.doi.org/10.1021/acs.jctc.1c00853
2021
1. Waigum, A., Black, J. A., & Köhn, A. “A generalized hybrid scheme for multireference methods.” The Journal of Chemical Physics 155, 204106 (2021)
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  @article{Waigum:JCP2022-204106, abstractnote = {A generalization of the hybrid scheme for multireference methods as recently put forward by Saitow and Yanai [J. Chem. Phys. 152, 114 111 (2020)] is presented. The hybrid methods are constructed by defining internal and external excitation spaces and evaluating these two subsets of excitations at different levels of theory. New hybrids that use the mix of internally contracted multireference coupled-cluster, unshifted multireference coupled electron pair, and multireference perturbation methods are derived and benchmarked. A new separation of the excitation space, which combines all singles and doubles excitations to the virtual orbitals into the external space, is also presented and tested. In general, the hybrid methods improve upon their non-hybrid parent method and offer a good compromise between computational complexity and numerical accuracy.}, author = {Waigum, A. and Black, J. A. and Köhn, A.}, doi = {10.1063/5.0067511}, issn = {0021-9606}, journal = {The Journal of Chemical Physics}, number = 20, pages = 204106, title = {A generalized hybrid scheme for multireference methods}, volume = 155, year = 2021 }
2. Schnabel, J., Cheng, L., & Köhn, A. “Limitations of perturbative coupled-cluster approximations for highly accurate investigations of Rb₂⁺.” J. Chem. Phys. 155, 124101 (2021)
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  @article{Schnabel_Cheng_Köhn_2021, abstractnote = {We reveal limitations of several standard coupled-cluster (CC) methods with perturbation-theory based noniterative or approximate iterative treatments of triple excitations when applied to the determination of highly accurate potential energy curves (PECs) of ionic dimers, such as the XΣg+2 electronic ground state of Rb2+. Such computations are of current interest for the understanding of ion–atom interactions in the ultracold regime. We demonstrate that these CC methods lead to an unphysical long-range barrier for the Rb2+ system. The barrier is small but spoils the long-range behavior of the PEC. The effect is also found for other X2+ systems, such as X = Li, Na, and K. Calculations using a flexible framework for obtaining leading perturbative triples corrections derived using an analytic CC singles and doubles energy derivative formulation demonstrate that the origin of this problem lies in the use of T̂3 amplitudes obtained from approximate CC singles, doubles, and triples amplitude equations. It is shown that the unphysical barrier is related to a symmetry instability of the underlying Hartree–Fock mean-field solution, leading to orbitals representing two +0.5-fold charged ions in the limit of separated fragments. This, in turn, leads to a wrong 1/R asymptote of the interaction potential computed by perturbation-based CC approximations. Physically meaningful perturbative corrections in the long-range tail of the PEC may instead be obtained using symmetry-broken reference determinants.}, author = {Schnabel, Jan and Cheng, Lan and Köhn, Andreas}, doi = {10.1063/5.0062098}, issn = {0021-9606}, journal = {J. Chem. Phys.}, pages = 124101, title = {Limitations of perturbative coupled-cluster approximations for highly accurate investigations of Rb₂⁺}, url = {https://dx.doi.org/10.1063/5.0062098}, volume = 155, year = 2021 }
  Link
  https://dx.doi.org/10.1063/5.0062098
3. Netz, J., Mitrushchenkov, A. O., & Köhn, A. “On the Accuracy of Mean-Field Spin–Orbit Operators for 3d Transition-Metal Systems.” Journal of Chemical Theory and Computation 17, 5530–5537 (2021)
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  @article{Netz_Mitrushchenkov_Köhn_2021, author = {Netz, Julia and Mitrushchenkov, Alexander O. and Köhn, Andreas}, doi = {10.1021/acs.jctc.1c00294}, issn = {1549-9618}, journal = {Journal of Chemical Theory and Computation}, pages = {5530–5537}, title = {On the Accuracy of Mean-Field Spin–Orbit Operators for 3d Transition-Metal Systems}, url = {https://dx.doi.org/10.1021/acs.jctc.1c00294}, volume = 17, year = 2021 }
  Link
  https://dx.doi.org/10.1021/acs.jctc.1c00294
4. Schnabel, J., Kampschulte, T., Rupp, S., Denschlag, J. H., & Köhn, A. “Towards photoassociation processes of ultracold rubidium trimers.” Phys. Rev. A 103, 022820 (2021)
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  @article{Schnabel_Kampschulte_Rupp_Denschlag_Köhn_2021, author = {Schnabel, Jan and Kampschulte, Tobias and Rupp, Simon and Denschlag, Johannes Hecker and Köhn, Andreas}, doi = {10.1103/physreva.103.022820}, issn = {2469-9926}, journal = {Phys. Rev. A}, pages = 022820, title = {Towards photoassociation processes of ultracold rubidium trimers}, url = {https://dx.doi.org/10.1103/physreva.103.022820}, volume = 103, year = 2021 }
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  https://dx.doi.org/10.1103/physreva.103.022820
2020
1. Köhn, A., Black, J. A., Aoto, Y. A., & Hanauer, M. “Improved and simplified orthogonalisation scheme and connected triples correction within the internally contracted multireference coupled-cluster method.” Molecular Physics 118, e1743889 (2020)
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  @article{Köhn_Black_Aoto_Hanauer_2020, author = {Köhn, Andreas and Black, Joshua A and Aoto, Yuri A and Hanauer, Matthias}, doi = {10.1080/00268976.2020.1743889}, journal = {Molecular Physics}, pages = {e1743889}, title = {Improved and simplified orthogonalisation scheme and connected triples correction within the internally contracted multireference coupled-cluster method}, url = {https://dx.doi.org/10.1080/00268976.2020.1743889}, volume = 118, year = 2020 }
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  https://dx.doi.org/10.1080/00268976.2020.1743889
2. Werner, H.-J., Knowles, P. J., Manby, F. R., Black, J. A., Doll, K., Heßelmann, A., Kats, D., Köhn, A., Korona, T., Kreplin, D. A., Ma, Q., Miller, T. F., Mitrushchenkov, A., Peterson, K. A., Polyak, I., Rauhut, G., & Sibaev, M. “The Molpro quantum chemistry package.” J. Chem. Phys. 152, 144107 (2020)
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  @article{doi:10.1063/5.0005081, author = {Werner, Hans-Joachim and Knowles, Peter J. and Manby, Frederick R. and Black, Joshua A. and Doll, Klaus and Heßelmann, Andreas and Kats, Daniel and Köhn, Andreas and Korona, Tatiana and Kreplin, David A. and Ma, Qianli and Miller, Thomas F. and Mitrushchenkov, Alexander and Peterson, Kirk A. and Polyak, Iakov and Rauhut, Guntram and Sibaev, Marat}, doi = {10.1063/5.0005081}, eprint = {https://doi.org/10.1063/5.0005081}, journal = {J. Chem. Phys.}, number = 14, pages = 144107, title = {The Molpro quantum chemistry package}, url = {https://doi.org/10.1063/5.0005081 }, volume = 152, year = 2020 }
  Link
  https://doi.org/10.1063/5.0005081
3. Clark, W. P., Köhn, A., & Niewa, R. “The Quasi-Binary Acetonitriletriide Sr₃C₂N₂.” Angew. Chem. Int. Ed. 59, 339–342 (2020)
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  @article{AK81, author = {Clark, William P and Köhn, Andreas and Niewa, Rainer}, doi = {10.1002/anie.201912831}, journal = {Angew. Chem. Int. Ed.}, language = {English}, number = 1, pages = {339-342}, title = {{The Quasi-Binary Acetonitriletriide Sr₃[C₂N]₂}}, url = {http://dx.doi.org/10.1002/anie.201912831}, volume = 59, year = 2020 }
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  http://dx.doi.org/10.1002/anie.201912831
2019
1. Forschner, R., Knelles, J., Bader, K., Müller, C., Frey, W., Köhn, A., Molard, Y., Giesselmann, F., & Laschat, S. “Flavylium Salts: A Blooming Core for Bioinspired Ionic Liquid Crystals.” Chem. Eur. J. 25, 12966–12980 (2019)
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  BibTeX
  @article{AK80, author = {Forschner, Robert and Knelles, Jakob and Bader, Korinna and M{\"u}ller, Carsten and Frey, Wolfgang and K{\"o}hn, Andreas and Molard, Yann and Giesselmann, Frank and Laschat, Sabine}, doi = {10.1002/chem.201901975}, journal = {Chem. Eur. J.}, language = {English}, number = 56, pages = {12966-12980}, title = {{Flavylium Salts: A Blooming Core for Bioinspired Ionic Liquid Crystals}}, url = {https://dx.doi.org/10.1002/chem.201901975}, volume = 25, year = 2019 }
  Link
  https://dx.doi.org/10.1002/chem.201901975
2. Black, J. A., & Köhn, A. “Linear and quadratic internally contracted multireference coupled-cluster approximations.” J. Chem. Phys. 150, 194107 (2019)
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  BibTeX
  @article{AK77, author = {Black, Joshua A. and Köhn, Andreas}, doi = {10.1063/1.5095070}, journal = {J. Chem. Phys.}, language = {English}, pages = 194107, title = {Linear and quadratic internally contracted multireference coupled-cluster approximations}, url = {https://dx.doi.org/10.1063/1.5095070}, volume = 150, year = 2019 }
  Link
  https://dx.doi.org/10.1063/1.5095070
3. Zens, A., Bauer, F., Kolb, B., Mannchen, F., Seubert, P., Forschner, R., Flaig, K. S., Köhn, A., Kunz, D., & Laschat, S. “Ni(NHC) Catalyzed Rearrangement of 1-Acyl-2-vinylcyclopropanes: Tackling a Mechanistic Puzzle by Combined Experimental and Computational Studies.” Eur. J. Org. Chem. 2019, 6285--6295 (2019)
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  BibTeX
  @article{AK79, author = {Zens, Anna and Bauer, Florian and Kolb, Benedikt and Mannchen, Fabian and Seubert, Philipp and Forschner, Robert and Flaig, Kim S and K{\"o}hn, Andreas and Kunz, Doris and Laschat, Sabine}, doi = {10.1002/ejoc.201901096}, journal = {Eur. J. Org. Chem.}, language = {English}, month = {09}, number = 36, pages = {6285--6295}, publisher = {John Wiley {\&} Sons, Ltd}, title = {{Ni(NHC) Catalyzed Rearrangement of 1-Acyl-2-vinylcyclopropanes: Tackling a Mechanistic Puzzle by Combined Experimental and Computational Studies}}, url = {http://dx.doi.org/10.1002/ejoc.201901096}, volume = 2019, year = 2019 }
  Link
  http://dx.doi.org/10.1002/ejoc.201901096
4. Kats, D., & Köhn, A. “On the distinguishable cluster approximation for triple excitations.” J. Chem. Phys. 150, 151101 (2019)
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  BibTeX
  @article{AK75, abstract = {The distinguishable cluster approximation applied to coupled cluster doubles equations greatly improves absolute and relative energies. We apply the same approximation to the triples equations and demonstrate that it can also improve the results of the coupled cluster method with singles, doubles, and triples. The resulting method has a nominal computational scaling of O(N7) in the real-space representation, and is orbital invariant, size extensive, and exact for three electrons.}, author = {Kats, Daniel and Köhn, Andreas}, doi = {10.1063/1.5096343}, journal = {J. Chem. Phys.}, pages = 151101, publisher = {AIP Publishing LLC}, title = {On the distinguishable cluster approximation for triple excitations}, url = {https://dx.doi.org/10.1063/1.5096343}, volume = 150, year = 2019 }
  Link
  https://dx.doi.org/10.1063/1.5096343
5. Aoto, Y. A., Bargholz, A., Kats, D., Werner, H.-J., & Köhn, A. “Perturbation Expansion of Internally Contracted Coupled-Cluster Theory up to Third Order.” J. Chem. Theory Comput. 15, 2291–2305 (2019)
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  BibTeX
  @article{AK74, author = {Aoto, Yuri Alexandre and Bargholz, Arne and Kats, Daniel and Werner, Hans-Joachim and Köhn, Andreas}, doi = {10.1021/acs.jctc.8b01301}, journal = {J. Chem. Theory Comput.}, pages = {2291-2305}, title = {Perturbation Expansion of Internally Contracted Coupled-Cluster Theory up to Third Order}, url = {https://dx.doi.org/10.1021/acs.jctc.8b01301}, volume = 15, year = 2019 }
  Link
  https://dx.doi.org/10.1021/acs.jctc.8b01301
6. Höfener, S., Schieschke, N., Klopper, W., & Köhn, A. “The extended explicitly-correlated second-order approximate coupled-cluster singles and doubles ansatz suitable for response theory.” J. Chem. Phys. 150, 184110 (2019)
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  BibTeX
  @article{AK76, author = {Höfener, Sebastian and Schieschke, Nils and Klopper, Wim and Köhn, Andreas}, doi = {10.1063/1.5094434}, journal = {J. Chem. Phys.}, language = {English}, pages = 184110, title = {The extended explicitly-correlated second-order approximate coupled-cluster singles and doubles ansatz suitable for response theory}, url = {https://dx.doi.org/10.1063/1.5094434}, volume = 150, year = 2019 }
  Link
  https://dx.doi.org/10.1063/1.5094434
7. Köhn, A., & Bargholz, A. “The second-order approximate internally contracted multireference coupled-cluster singles and doubles method icMRCC2.” J. Chem. Phys. 151, 041106 (2019)
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  BibTeX
  @article{AK78, author = {Köhn, Andreas and Bargholz, Arne}, doi = {10.1063/1.5115045}, journal = {J. Chem. Phys.}, pages = 041106, publisher = {AIP Publishing, LLC}, title = {The second-order approximate internally contracted multireference coupled-cluster singles and doubles method icMRCC2}, url = {https://doi.org/10.1063/1.5115045}, volume = 151, year = 2019 }
  Link
  https://doi.org/10.1063/1.5115045
2018
1. Liu, W., Canola, S., Köhn, A., Engels, B., Negri, F., & Fink, R. F. “A model Hamiltonian tuned toward high level ab initiocalculations to describe the character of excitonic states in perylenebisimide aggregates.” J. Comput. Chem. 39, 1979–1989 (2018)
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  BibTeX
  @article{Liu:2018dv, author = {Liu, Wenlan and Canola, Sofia and K{\"o}hn, Andreas and Engels, Bernd and Negri, Fabrizia and Fink, Reinhold F}, doi = {10.1002/jcc.25374}, journal = {J. Comput. Chem.}, language = {English}, month = {10}, number = 24, pages = {1979-1989}, publisher = {John Wiley {\&} Sons, Ltd}, title = {{A model Hamiltonian tuned toward high level ab initiocalculations to describe the character of excitonic states in perylenebisimide aggregates}}, url = {https://dx.doi.org/10.1002/jcc.25374}, volume = 39, year = 2018 }
  Link
  https://dx.doi.org/10.1002/jcc.25374
2. Kesharwani, M. K., Sylvetsky, N., Köhn, A., Tew, D. P., & Martin, J. M. L. “Do CCSD and approximate CCSD-F12 variants converge to the same basis set limits? the case of atomization energies.” J. Chem. Phys. 149, 154109 (2018)
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  @article{AK72, abstract = {While the title question is a clear 'yes' from purely theoretical arguments, the case is less clear for practical calculations with finite (one-particle) basis sets. To shed further light on this issue, the basis set limits of CCSD (coupled cluster theory with all single and double excitations) and of different approximate implementations of CCSD-F12 (explicitly correlated CCSD) have been investigated on detail for the W4-17 thermochemical benchmark. Near the CBS (［1-particle］ complete basis set) limit, CCSD and CCSD(F12*) agree to within their respective uncertainties (about $\backslash$pm0.04 kcal/mol) due to residual basis set incompleteness error, but a nontrivial difference remains between CCSD-F12b and CCSD(F12*), which is roughly proportional to the degree of static correlation. The observed basis set convergence behavior results from the superposition of a rapidly converging, attractive, CCSD［F12］–CCSD-F12b difference (consisting mostly of third-order terms), and a more slowly converging, repulsive, fourth-order difference between CCSD(F12*) and CCSD［F12］. For accurate thermochemistry, we recommend CCSD(F12*) over CCSD-F12b if at all possible. There are some indications that the nZaPa family of basis sets exhibits somewhat smoother convergence than the correlation consistent family}, archiveprefix = {arXiv}, arxivid = {1807.06058}, author = {Kesharwani, Manoj K. and Sylvetsky, Nitai and K{\"{o}}hn, Andreas and Tew, David P. and Martin, Jan M.L.}, doi = {10.1063/1.5048665}, eprint = {1807.06058}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 15, pages = 154109, title = {{Do CCSD and approximate CCSD-F12 variants converge to the same basis set limits? the case of atomization energies}}, url = {http://dx.doi.org/10.1063/1.5048665}, volume = 149, year = 2018 }
  Link
  http://dx.doi.org/10.1063/1.5048665
3. Coughtrie, D. J., Giereth, R., Kats, D., Werner, H. J., & Köhn, A. “Embedded Multireference Coupled Cluster Theory.” J. Chem. Theory Comput. 14, 693–709 (2018)
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  @article{AK70, abstract = {Internally contracted multireference coupled cluster (icMRCC) theory is embedded within multireference perturbation theory (MRPT) to calculate energy differences in large strongly correlated systems. The embedding scheme is based on partitioning the orbital spaces of a complete active space self-consistent field (CASSCF) wave function, with a truncated virtual space constructed by transforming selected projected atomic orbitals (PAOs). MRPT is applied to the environment using a subtractive embedding approach that also allows for multilayer embedding. Benchmark calculations are presented for biradical bond dissociation, spin splitting in a heterocyclic carbene and hydrated Fe(II), and for the super-exchange coupling constant in solid nickel oxide. The method is further applied to two large transition metal complexes with a triple-{\$}\zeta{\$} basis set: an iron complex with 175 atoms and 2939 basis functions, and a nickel complex with 231 atoms, and 4175 basis functions.}, author = {Coughtrie, David J. and Giereth, Robin and Kats, Daniel and Werner, Hans Joachim and K{\"{o}}hn, Andreas}, doi = {10.1021/acs.jctc.7b01144}, issn = {15499626}, journal = {J. Chem. Theory Comput.}, number = 2, pages = {693–709}, title = {{Embedded Multireference Coupled Cluster Theory}}, url = {http://dx.doi.org/10.1021/acs.jctc.7b01144}, volume = 14, year = 2018 }
  Link
  http://dx.doi.org/10.1021/acs.jctc.7b01144
4. Samanta, P. K., & Köhn, A. “First-order properties from internally contracted multireference coupled-cluster theory with particular focus on hyperfine coupling tensors.” J. Chem. Phys. 149, 64101 (2018)
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  @article{AK71, abstract = {Internally contracted multireference coupled-cluster (icMRCC) theory is extended to the computation of first-order properties (expectation values). We use the previously defined Lagrange formulation of the energy functional to derive the required equations for the Lagrange multipliers and arrive at an expression for first-order properties according to the generalized Hellmann-Feynman theorem, analogous to single-reference coupled-cluster theory. The present formulation does not include orbital relaxation, but in line with previous experience in coupled-cluster theory, the single-excitation cluster operator can recover a significant portion of orbital relaxation. Further aspects of the theory that arise from the internal contraction approach are discussed. Using automated derivation techniques, we have implemented a pilot code for icMRCCSD and icMRCCSDT for testing the method numerically. We find good agreement with full configuration interaction for several properties of boron monohydride and dipole moment curves of hydrogen fluoride and chromium hydride. A particular focus is given to spin-dependent properties: The hyperfine coupling tensors of Σ and Π radicals have been computed and compared to experiment and previous computations. We discuss the problem of describing spin polarization with properly spin-adapted wavefunctions, which requires either including pseudo-triple excitations or employing sufficiently flexible reference functions. Published by AIP Publishing. https://doi.org/10.1063/1.5040587}, author = {Samanta, Pradipta Kumar and K{\"{o}}hn, Andreas}, doi = {10.1063/1.5040587}, isbn = {6578138390}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 6, pages = 64101, pmid = {30111121}, title = {{First-order properties from internally contracted multireference coupled-cluster theory with particular focus on hyperfine coupling tensors}}, url = {http://dx.doi.org/10.1063/1.5040587}, volume = 149, year = 2018 }
  Link
  http://dx.doi.org/10.1063/1.5040587
5. Liu, W., Müller, L., Ma, S., Barlow, S., Marder, S. R., Kowalsky, W., Köhn, A., & Lovrincic, R. “The Origin of the π-π Spacing Change upon Doping of Semiconducting Polymers.” J. Phys. Chem. C 122, 27983–27990 (2018)
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  BibTeX
  @article{AK73, abstract = {While there is agreement about the local structural order of semiconducting polymers such as poly(3-hexylthiophene) (P3HT), there is still debate over the impact of molecular doping. One prevalent interpretation is that dopant molecules intercalate in the π-π stacking of crystallites; however, this idea has recently been challenged. We present here electron-diffraction measurements of P3HT doped with the two dopants 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and molybdenum tris［1-(methoxycarbonyl)-2-(trifluoromethyl)-ethane-1,2-dithiolene］ (Mo(tfd-CO2Me)3), which have considerably different sizes and shapes, processed by different doping techniques. We observe a reduction in the π-π spacing of P3HT upon doping with both dopant molecules and doping techniques. These data are not consistent with both of the dopants intercalating in the π-π stacks and an alternative explanation is, therefore, required to explain these results. Density functional theory calculations for P3HT model oligomers...}, author = {Liu, Wenlan and M{\"{u}}ller, Lars and Ma, Shuangying and Barlow, Stephen and Marder, Seth R. and Kowalsky, Wolfgang and K{\"{o}}hn, Andreas and Lovrincic, Robert}, doi = {10.1021/acs.jpcc.8b10845}, issn = {1932-7447}, journal = {J. Phys. Chem. C}, pages = {27983-27990}, title = {{The Origin of the π-π Spacing Change upon Doping of Semiconducting Polymers}}, url = {http://dx.doi.org/10.1021/acs.jpcc.8b10845}, volume = 122, year = 2018 }
  Link
  http://dx.doi.org/10.1021/acs.jpcc.8b10845
2017
1. Hietzschold, S., Hillebrandt, S., Ullrich, F., Bombsch, J., Rohnacher, V., Ma, S., Liu, W., Köhn, A., Jaegermann, W., Pucci, A., Kowalsky, W., Mankel, E., Beck, S., & Lovrincic, R. “Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity.” ACS Appl. Mater. Interfaces 9, 39821–39829 (2017)
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  @article{AK68, abstract = {Nickel oxide (NiO) is a widely used material for efficient hole extraction in optoelectronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solution-processed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives an insight into the chemical composition of the SAM. Density functional theory calculations reveal the possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of...}, author = {Hietzschold, Sebastian and Hillebrandt, Sabina and Ullrich, Florian and Bombsch, Jakob and Rohnacher, Valentina and Ma, Shuangying and Liu, Wenlan and K{\"{o}}hn, Andreas and Jaegermann, Wolfram and Pucci, Annemarie and Kowalsky, Wolfgang and Mankel, Eric and Beck, Sebastian and Lovrincic, Robert}, doi = {10.1021/acsami.7b12784}, issn = {19448252}, journal = {ACS Appl. Mater. Interfaces}, number = 45, pages = {39821–39829}, title = {{Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity}}, url = {http://dx.doi.org/10.1021/acsami.7b12784}, volume = 9, year = 2017 }
  Link
  http://dx.doi.org/10.1021/acsami.7b12784
2. Aoto, Y. A., De Lima Batista, A. P., Köhn, A., & De Oliveira-Filho, A. G. S. “How to Arrive at Accurate Benchmark Values for Transition Metal Compounds: Computation or Experiment?” J. Chem. Theory Comput. 13, 5291–5316 (2017)
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  @article{AK66, abstract = {With the objective of analyzing which kind of reference data is appropriate for benchmarking quantum chemical approaches for transition metal compounds, we present the following, (a) a collection of 60 transition metal diatomic molecules for which experimentally derived With the objective of analyzing which kind of reference data is appropriate for benchmarking quantum chemical approaches for transition metal compounds, we present the following, (a) a collection of 60 transition metal diatomic molecules for which experimentally derived dissoci- ation energies, equilibrium distances, and harmonic vibrational frequencies are known and (b) a composite computational approach based on coupled-cluster theory with basis set extra- polation, inclusion of core−valence correlation, and correc- tions for relativistic and multireference effects. The latter correction was obtained from internally contracted multi- reference coupled-cluster (icMRCC) theory. This composite approach has been used to obtain the dissociation energies and spectroscopic constants for the 60 molecules in our data set. In accordance with previous studies on a subset of molecules, we find that multireference corrections are rather small in many cases and CCSD(T) can provide accurate reference values, if the complete basis set limit is explored. In addition, the multireference correction improves the results in cases where CCSD(T) is not a good approximation. For a few cases, however, strong deviations from experiment persist, which cannot be explained by the remaining error in the computational approach. We suggest that these experimentally derived values require careful revision. This also shows that reliable reference values for benchmarking approximate computational methods are not always easily accessible via experiment and accurate computations may provide an alternative way to access them. In order to assess how the choice of reference data affects benchmark studies, we tested 10 DFT functionals for the molecules in the present data set against experimental and calculated reference values. Despite the differences between these two sets of reference values, we found that the ranking of the relative performance of the DFT functionals is nearly independent of the chosen reference.}, author = {Aoto, Yuri A. and {De Lima Batista}, Ana Paula and K{\"{o}}hn, Andreas and {De Oliveira-Filho}, Antonio G.S.}, doi = {10.1021/acs.jctc.7b00688}, issn = {15499626}, journal = {J. Chem. Theory Comput.}, number = 11, pages = {5291–5316}, title = {{How to Arrive at Accurate Benchmark Values for Transition Metal Compounds: Computation or Experiment?}}, url = {http://dx.doi.org/10.1021/acs.jctc.7b00688}, volume = 13, year = 2017 }
  Link
  http://dx.doi.org/10.1021/acs.jctc.7b00688
3. Lipparini, F., Kirsch, T., Köhn, A., & Gauss, J. “Internally Contracted Multireference Coupled Cluster Calculations with a Spin-Free Dirac-Coulomb Hamiltonian: Application to the Monoxides of Titanium, Zirconium, and Hafnium.” J. Chem. Theory Comput. 13, 3171–3184 (2017)
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  @article{AK64, abstract = {In this contribution, we combine internally-contracted multireference coupled clus- ter theory with a four-component treatment of scalar-relativistic effects based on the spin-free Dirac-Coulomb Hamiltonian. This strategy allows for a rigorous treatment of static and dynamic correlation as well as scalar-relativistic effects, which makes it vi- able to describe molecules containing heavy transition elements. The use of a spin-free formalism limits the impact of the four-component treatment on the computational cost to the non rate-determining steps of the calculations. We apply the newly de- veloped method to the lowest singlet and triplet states of the monoxides of titanium, zirconium, and hafnium and show how the interplay between electronic correlation and relativistic effects explains the electronic structure of such molecules.}, author = {Lipparini, Filippo and Kirsch, Till and K{\"{o}}hn, Andreas and Gauss, J{\"{u}}rgen}, doi = {10.1021/acs.jctc.7b00110}, issn = {15499626}, journal = {J. Chem. Theory Comput.}, number = 7, pages = {3171–3184}, title = {{Internally Contracted Multireference Coupled Cluster Calculations with a Spin-Free Dirac-Coulomb Hamiltonian: Application to the Monoxides of Titanium, Zirconium, and Hafnium}}, url = {http://dx.doi.org/10.1021/acs.jctc.7b00110}, volume = 13, year = 2017 }
  Link
  http://dx.doi.org/10.1021/acs.jctc.7b00110
4. Haenle, J. C., Bruchlos, K., Ludwigs, S., Köhn, A., & Laschat, S. “Rigidified Push–Pull Dyes: Using Chromophore Size, Donor, and Acceptor Units to Tune the Ground State between Neutral and the Cyanine Limit.” Chempluschem 82, 1197–1210 (2017)
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  @article{AK65, abstract = {{\textcopyright} 2017 Wiley-VCH Verlag GmbH {\&} Co. KGaA, Weinheim. A series of rigidified dyes with either dodecyloxy or piperidyl donors in combination with either malononitrile or barbiturate acceptors were synthesized. To analyze the influence of varying the donor (piperidyl vs. dodecyloxy), chromophore (mono-, bis-, or tricyclic system), and acceptor moiety (methyl barbiturate vs. malononitrile) on the ground-state polarity, a combination of several methods was employed: UV/Vis absorption and emission, quantum chemical computations, 1 H as well as 13 CNMR spectroscopy, and cyclic voltammetry measurements. Depending on the acceptor and donor moieties, the ground state was shifted from a neutral form to the cyanine limit and further to a zwitterionic structure. When the dye had strong piperidyl donor and barbiturate acceptor substituents, the ground state was close to the cyanine limit, which resulted in strong cyanine-like absorption properties with a dominant 0-0 transition. When the dye combined a weak dodecyloxy donor with a malononitrile or barbiturate acceptor, the neutral resonance form significantly contributed to the ground state, leading to weaker but broader absorption spectra featuring transitions to higher vibronic states.}, author = {Haenle, Johannes Christian and Bruchlos, Kirsten and Ludwigs, Sabine and K{\"{o}}hn, Andreas and Laschat, Sabine}, doi = {10.1002/cplu.201700347}, issn = {21926506}, journal = {Chempluschem}, number = 9, pages = {1197–1210}, title = {{Rigidified Push–Pull Dyes: Using Chromophore Size, Donor, and Acceptor Units to Tune the Ground State between Neutral and the Cyanine Limit}}, url = {http://dx.doi.org/10.1002/cplu.201700347}, volume = 82, year = 2017 }
  Link
  http://dx.doi.org/10.1002/cplu.201700347
5. Mühlhäuser, T., Savin, A., Frey, W., Baro, A., Schneider, A. J., Döteberg, H. G., Bauer, F., Köhn, A., & Laschat, S. “Role of Regioisomeric Bicyclo［3.3.0］octa-2,5-diene Ligands in Rh Catalysis: Synthesis, Structural Analysis, Theoretical Study, and Application in Asymmetric 1,2- and 1,4-Additions.” J. Org. Chem. 82, 13468–13480 (2017)
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  @article{AK69, author = {M{\"{u}}hlh{\"{a}}user, Tina and Savin, Alex and Frey, Wolfgang and Baro, Angelika and Schneider, Andreas J. and D{\"{o}}teberg, Heinz G{\"{u}}nter and Bauer, Florian and K{\"{o}}hn, Andreas and Laschat, Sabine}, doi = {10.1021/acs.joc.7b02601}, issn = {15206904}, journal = {J. Org. Chem.}, number = 24, pages = {13468–13480}, title = {{Role of Regioisomeric Bicyclo［3.3.0］octa-2,5-diene Ligands in Rh Catalysis: Synthesis, Structural Analysis, Theoretical Study, and Application in Asymmetric 1,2- and 1,4-Additions}}, url = {http://dx.doi.org/10.1021/acs.joc.7b02601}, volume = 82, year = 2017 }
  Link
  http://dx.doi.org/10.1021/acs.joc.7b02601
6. Mai, S., Plasser, F., Pabst, M., Neese, F., Köhn, A., & González, L. “Surface hopping dynamics including intersystem crossing using the algebraic diagrammatic construction method.” J. Chem. Phys. 147, 184109 (2017)
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  @article{AK67, abstract = {We report an implementation for employing the algebraic diagrammatic construction to second order ［ADC(2)］ ab initio electronic structure level of theory in nonadiabatic dynamics simulations in the framework of the SHARC (surface hopping including arbitrary couplings) dynamics method. The implementation is intended to enable computationally efficient, reliable, and easy-to-use nonadiabatic dynamics simulations of intersystem crossing in organic molecules. The methodology is evaluated for the 2-thiouracil molecule. It is shown that ADC(2) yields reliable excited-state energies, wave functions, and spin-orbit coupling terms for this molecule. Dynamics simulations are compared to previously reported results using high-level multi-state complete active space perturbation theory, showing favorable agreement. Published by AIP Publishing. https://doi.org/10.1063/1.4999687}, author = {Mai, Sebastian and Plasser, Felix and Pabst, Mathias and Neese, Frank and K{\"{o}}hn, Andreas and Gonz{\'{a}}lez, Leticia}, doi = {10.1063/1.4999687}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 18, pages = 184109, title = {{Surface hopping dynamics including intersystem crossing using the algebraic diagrammatic construction method}}, url = {http://dx.doi.org/10.1063/1.4999687}, volume = 147, year = 2017 }
  Link
  http://dx.doi.org/10.1063/1.4999687
7. Wöhrle, T., Gündemir, R., Frey, W., Knecht, F., Köhn, A., & Laschat, S. “Thermotropic MIDA Boronates as a Case Study for the Role of Dipolar Interactions in Liquid Crystalline Self-Assembly.” Chem. - A Eur. J. 23, 4149–4159 (2017)
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  @article{AK63, abstract = {A series of MIDA (N-methylimino diacetic acid) boronates carrying 4-alkoxy, 3,4-bisalkoxy, or 3,4,5-trisalkoxyphenyl substituents were synthesized and their mesomorphic properties characterized by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) techniques such as small- and wide-angle X-ray scattering (SAXS and WAXS, respectively). Most derivatives were liquid crystalline. In the case of mono- and bisalkoxy-substituted derivatives, C6 chains already induced smectic A (SmA) mesophases despite the bulky MIDA head group. With increasing chain length, columnar hexagonal (Colh) phases replaced SmA phases in the disubstituted series. Quantum chemical calculations on a series of MIDA boronates show that the B−N bond is a dative bond with a positive charge on the boron atom and negative charges on the nitrogen and oxygen atoms. In addition, no π-interaction between the aryl moiety and B−N bond was found, thus the mesogenic unit is electronically decoupled from the MIDA head group. These theoretical findings were supported by IR and Raman spectra as well as by asingle crystal structure analysis of 4-ethoxyphenyl MIDA boronate. Calculations of the electrostatic potential of the MIDA boronate reveal a special polarity pattern that can support the formation of a two-dimensional network and is likely to explain the liquid crystalline self-assembly. The absence of any electronic cross-talk between the MIDA head group and B-aryl or B-alkyl substituents allows the efficient tailoring of the mesophase type through variation of the substituents.}, author = {W{\"{o}}hrle, Tobias and G{\"{u}}ndemir, Rafet and Frey, Wolfgang and Knecht, Friederike and K{\"{o}}hn, Andreas and Laschat, Sabine}, doi = {10.1002/chem.201605648}, isbn = {0947-6539}, issn = {15213765}, journal = {Chem. - A Eur. J.}, number = 17, pages = {4149–4159}, title = {{Thermotropic MIDA Boronates as a Case Study for the Role of Dipolar Interactions in Liquid Crystalline Self-Assembly}}, url = {http://dx.doi.org/10.1002/chem.201605648}, volume = 23, year = 2017 }
  Link
  http://dx.doi.org/10.1002/chem.201605648
2016
1. Kühn, M., Mankel, E., Köhn, A., Mayer, T., & Jaegermann, W. “Doping mechanism of MoO₃ in 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl: A photoelectron spectroscopic study.” Phys. Status Solidi Basic Res. 253, 1697–1706 (2016)
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  @article{AK60, abstract = { The doping mechanism of molybdenum oxide (MoO₃) in the hole transport material 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) is explored extensively using photoelectron spectroscopy. Doping limit and doping efficiency are investigated in dependence on the MoO₃ content. In addition, shape changes in Mo3d and C1s core level spectra of doped layers are compared to the respective emissions of the pristine materials. Using density functional theory (DFT) calculations, the additional feature in the C1s emission can be assigned to CBP+ cations and in the Mo3d emission a Mo5+ species can be identified. Therefore, charge transfer can not only be deduced indirectly from the movement of the Fermi level within the CBP energy gap, but also directly in the formation of cations of the matrix and anionic dopant species. Finally, the dopant morphology in dependence on the MoO₃ content is derived indirectly from XPS measurements and is correlated to the observed doping efficiency. Because MoO₃ precipitates form in the CBP matrix as evident from TEM measurements, the bilayer interface band alignment is analyzed by stepwise evaporation of CBP onto a MoO₃ layer. The results of the interface experiment show that the CBP:MoO₃ doping behavior can be well understood with the internal interface charge transfer doping model. }, author = {K{\"{u}}hn, Maybritt and Mankel, Eric and K{\"{o}}hn, Andreas and Mayer, Thomas and Jaegermann, Wolfram}, doi = {10.1002/pssb.201600144}, issn = {15213951}, journal = {Phys. Status Solidi Basic Res.}, number = 9, pages = {1697–1706}, title = {{Doping mechanism of MoO₃ in 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl: A photoelectron spectroscopic study}}, url = {http://dx.doi.org/10.1002/pssb.201600144}, volume = 253, year = 2016 }
  Link
  http://dx.doi.org/10.1002/pssb.201600144
2. Aoto, Y. A., & Köhn, A. “Internally contracted multireference coupled-cluster theory in a multistate framework.” J. Chem. Phys. 144, 74103 (2016)
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  @article{AK58, abstract = {The internally contracted multireference coupled cluster (icMRCC) theory is reexamined in a multistate framework. The new derivation starts from the Bloch equations employing a wave operator similar to the one originally employed by Jeziorski and Monkhorst ［Phys. Rev. A 24, 1668 (1981)］, but allows for a multideterminantal model-space basis. The resulting working equations are a generalization of state-universal multireference coupled-cluster (SU-MRCC) theory. In the case of truncated cluster operators, energies and wave functions depend on the choice of the model-space basis. It is shown that the state-specific solutions of the original icMRCC theory exactly diagonalize the effective Hamiltonian of the multistate theory and thus decouple the multistate equations. While this in principle means that icMRCC is a multistate theory, we find that truncated icMRCC theory still shows minor artifacts close to avoided crossings, which can be removed by considering a true multistate ansatz. The results also shed new light on the orbital variance of the original SU-MRCC theory.}, author = {Aoto, Yuri Alexandre and K{\"{o}}hn, Andreas}, doi = {10.1063/1.4941604}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 7, pages = 74103, title = {{Internally contracted multireference coupled-cluster theory in a multistate framework}}, url = {http://dx.doi.org/10.1063/1.4941604}, volume = 144, year = 2016 }
  Link
  http://dx.doi.org/10.1063/1.4941604
3. Lamberts, T., Samanta, P. K., Köhn, A., & Kästner, J. “Quantum tunneling during interstellar surface-catalyzed formation of water: The reaction H + H₂O₂→ H₂O + OH.” Phys. Chem. Chem. Phys. 18, 33021–33030 (2016)
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  @article{AK62, abstract = {The final step of the water formation network on interstellar grain surfaces starting from the H + O2 route is the reaction between H and H₂O₂. This reaction is known to have a high activation energy and therefore at low temperatures it can only proceed via tunneling. To date, however, no rate constants are available at temperatures below 200 K. In this work, we use instanton theory to compute rate constants for the title reaction with and without isotopic substitutions down to temperatures of 50 K. The calculations are based on density functional theory, with additional benchmarks for the activation energy using unrestricted single-reference and multireference coupled-cluster single-point energies. Gas-phase bimolecular rate constants are calculated and compared with available experimental data not only for H + H₂O₂ → H₂O + OH, but also for H + H₂O₂ → H₂ + HO2. We find a branching ratio where the title reaction is favored by at least two orders of magnitude at 114 K. In the interstellar medium this reaction predominantly occurs on water surfaces, which increases the probability that the two reactants meet. To mimic this, one, two, or three spectator H₂O molecules are added to the system. Eley–Rideal bimolecular and Langmuir–Hinshelwood unimolecular rate constants are presented here. The kinetic isotope effects for the various cases are compared to experimental data as well as to expressions commonly used in astrochemical models. Both the rectangular barrier and the Eckart approximations lead to errors of about an order of magnitude. Finally, fits of the rate constants are provided as input for astrochemical models}, archiveprefix = {arXiv}, arxivid = {1611.09188}, author = {Lamberts, Thanja and Samanta, Pradipta Kumar and K{\"{o}}hn, Andreas and K{\"{a}}stner, Johannes}, doi = {10.1039/C6CP06457D}, eprint = {1611.09188}, issn = {14639076}, journal = {Phys. Chem. Chem. Phys.}, number = 48, pages = {33021–33030}, title = {{Quantum tunneling during interstellar surface-catalyzed formation of water: The reaction H + H₂O₂→ H₂O + OH}}, url = {http://dx.doi.org/10.1039/C6CP06457D}, volume = 18, year = 2016 }
  Link
  http://dx.doi.org/10.1039/C6CP06457D
4. Aoto, Y. A., & Köhn, A. “Revisiting the F + HCl → HF + Cl reaction using a multireference coupled-cluster method.” Phys. Chem. Chem. Phys. 18, 30241–30253 (2016)
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  @article{AK61, abstract = {A potential energy surface for the title reaction is constructed using a multireference coupled-cluster method, giving rate constant in excellent agreement with experiments}, author = {Aoto, Yuri Alexandre and K{\"{o}}hn, Andreas}, doi = {10.1039/c6cp05782a}, issn = {14639076}, journal = {Phys. Chem. Chem. Phys.}, number = 44, pages = {30241–30253}, title = {{Revisiting the F + HCl → HF + Cl reaction using a multireference coupled-cluster method}}, url = {http://dx.doi.org/10.1039/c6cp05782a}, volume = 18, year = 2016 }
  Link
  http://dx.doi.org/10.1039/c6cp05782a
5. Loke, I., Bentzinger, G., Holz, J., Raja, A., Bhasin, A., Sasse, F., Köhn, A., Schobert, R., & Laschat, S. “Synthesis of the AB ring system of clifednamide utilizing Claisen rearrangement and Diels-Alder reaction as key steps.” Org. Biomol. Chem. 14, 884–894 (2016)
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  @article{AK59, abstract = {In order to construct the functionalized AB ring system of clifednamide, member of the class of macrocyclic tetramic acid lactams, a synthesis was developed which utilized an Ireland-Claisen rearrangement and an intramol. Diels-Alder reaction. Starting from di-O-isopropylidene-D-mannitol the allyl carboxylate precursor for the sigmatropic rearrangement was prepd. This rearrangement proceeded diastereoselectively only in the presence of an allyl silyl ether instead of the parent enone in the side chain, as suggested by deuteration expts. A subsequent Diels-Alder reaction yielded the target Et hexahydro-1H-indene-carboxylate with high diastereoselectivity. Quantum-chem. investigations of this intramol. Diels-Alder reaction support the proposed configuration of the final product. ［on SciFinder(R)］}, author = {Loke, Inga and Bentzinger, Guillaume and Holz, Julia and Raja, Aruna and Bhasin, Aman and Sasse, Florenz and K{\"{o}}hn, Andreas and Schobert, Rainer and Laschat, Sabine}, doi = {10.1039/c5ob01491c}, issn = {14770520}, journal = {Org. Biomol. Chem.}, number = 3, pages = {884–894}, title = {{Synthesis of the AB ring system of clifednamide utilizing Claisen rearrangement and Diels-Alder reaction as key steps}}, url = {http://dx.doi.org/10.1039/c5ob01491c}, volume = 14, year = 2016 }
  Link
  http://dx.doi.org/10.1039/c5ob01491c
2015
1. Liu, W., Lunkenheimer, B., Settels, V., Engels, B., Fink, R. F., & Köhn, A. “A general ansatz for constructing quasi-diabatic states in electronically excited aggregated systems.” J. Chem. Phys. 143, 84106 (2015)
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  @article{AK56, abstract = {We present a general method for analyzing the character of singly excited states in terms of charge transfer (CT) and locally excited (LE) configurations. The analysis is formulated for configuration interaction singles (CIS) singly excited wave functions of aggregate systems. It also approximately works for the second-order approximate coupled cluster singles and doubles and the second-order algebraic-diagrammatic construction methods ［CC2 and ADC(2)］. The analysis method not only generates a weight of each character for an excited state, but also allows to define the related quasi-diabatic states and corresponding coupling matrix elements. In the character analysis approach, we divide the target system into domains and use a modified Pipek-Mezey algorithm to localize the canonical MOs on each domain, respectively. The CIS wavefunction is then transformed into the localized basis, which allows us to partition the wavefunction into LE configurations within domains and CT configuration between pairs of different domains. Quasi-diabatic states are then obtained by mixing excited states subject to the condition of maximizing the weight of one single LE or CT configuration (localization in configuration space). Different aims of such a procedure are discussed, either the construction of pure LE and CT states for analysis purposes (by including a large number of excited states) or the construction of effective models for dynamics calculations (by including a restricted number of excited states). Applications are given to LE/CT mixing in -stacked systems, charge-recombination matrix elements in a hetero-dimer, and excitonic couplings in multi-chromophoric systems.}, author = {Liu, Wenlan and Lunkenheimer, Bernd and Settels, Volker and Engels, Bernd and Fink, Reinhold F. and K{\"{o}}hn, Andreas}, doi = {10.1063/1.4929352}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 8, pages = 84106, pmid = {26328817}, title = {{A general ansatz for constructing quasi-diabatic states in electronically excited aggregated systems}}, url = {http://dx.doi.org/10.1063/1.4929352}, volume = 143, year = 2015 }
  Link
  http://dx.doi.org/10.1063/1.4929352
2. Mondal, S., Mandal, S., Carrella, L., Jana, A., Fleck, M., Köhn, A., Rentschler, E., & Mohanta, S. “A series of MIICuII3stars (M = Mn, Ni, Cu, Zn) exhibiting unusual magnetic properties.” Inorg. Chem. 54, 117–131 (2015)
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  @article{AK53, abstract = {The work in this report describes the syntheses, electrospray ionization mass spectrometry, structures, and exptl. and d. functional theor. (DFT) magnetic properties of four tetrametallic stars of compn. ［MII(CuIIL)3］(ClO4)2 (1, M = Mn; 2, M = Ni; 3, M = Cu; 4, M = Zn) derived from a single-compartment Schiff base ligand, N,N'-bis(salicylidene)-1,4-butanediamine (H₂L), which is the ［2 + 1］ condensation product of salicylaldehyde and 1,4-diaminobutane. The central metal ion (MnII, NiII, CuII, or ZnII) is linked with two μ2-phenoxo bridges of each of the three ［CuIIL］ moieties, and thus the central metal ion is encapsulated in between three ［CuIIL］ units. The title compds. are rare or sole examples of stars having these metal-ion combinations. In the cases of 1, 3, and 4, the four metal ions form a centered isosceles triangle, while the four metal ions in 2 form a centered equilateral triangle. Both the variable-temp. magnetic susceptibility and variable-field magnetization (at 2-10 K) of 1-3 were measured and simulated contemporaneously. While the MnIICuII3 compd. 1 exhibits ferromagnetic interaction with J = 1.02 cm-1, the NiIICuII3 compd. 2 and CuIICuII3 compd. 3 exhibit antiferromagnetic interaction with J = -3.53 and -35.5 cm-1, resp. Variable-temp. magnetic susceptibility data of the ZnIICuII3 compd. 4 indicate very weak antiferromagnetic interaction of -1.4 cm-1, as expected. On the basis of known correlations, the magnetic properties of 1-3 are unusual; it seems that ferromagnetic interaction in 1 and weak/moderate antiferromagnetic interaction in 2 and 3 are possibly related to the distorted coordination environment of the peripheral copper(II) centers (intermediate between square-planar and tetrahedral). DFT calcns. were done to elucidate the magnetic properties. The DFT-computed J values are quant. (for 1) or qual. (for 2 and 3) matched well with the exptl. values. Spin densities and magnetic orbitals (natural bond orbitals) correspond well with the trend of obsd./computed magnetic exchange interactions. ［on SciFinder(R)］}, author = {Mondal, Suraj and Mandal, Shuvankar and Carrella, Luca and Jana, Arpita and Fleck, Michel and K{\"{o}}hn, Andreas and Rentschler, Eva and Mohanta, Sasankasekhar}, doi = {10.1021/ic501900d}, isbn = {0020-1669}, issn = {1520510X}, journal = {Inorg. Chem.}, number = 1, pages = {117–131}, title = {{A series of MIICuII3stars (M = Mn, Ni, Cu, Zn) exhibiting unusual magnetic properties}}, url = {http://dx.doi.org/10.1021/ic501900d}, volume = 54, year = 2015 }
  Link
  http://dx.doi.org/10.1021/ic501900d
3. Georgieva, I., Aquino, A. J. A., Plasser, F., Trendafilova, N., Köhn, A., & Lischka, H. “Intramolecular Charge-Transfer Excited-State Processes in 4-(N, N -Dimethylamino)benzonitrile: The Role of Twisting and the πσ∗ State.” J. Phys. Chem. A 119, 6232–6243 (2015)
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  @article{AK54, abstract = {Small-scale portable rainfall simulators are an essential research tool for investigating the process dynamics of soil erosion and surface hydrology. There is no standardisation of rainfall simulation and such rainfall simulators differ in design, rainfall intensities, rain spectra and research questions, which impede drawing a meaningful comparison between results. Nevertheless, these data become progressively important for soil erosion assessment and therefore, the basis for decision-makers in application-oriented erosion protection.The artificially generated rainfall of the simulators used at the Universities Basel, La Rioja, Malaga, Trier, T??bingen, Valencia, Wageningen, Zaragoza, and at different CSIC (Spanish Scientific Research Council) institutes (Almeria, Cordoba, Granada, Murcia and Zaragoza) was measured with the same methods (Laser Precipitation Monitor for drop spectra and rain collectors for spatial distribution). Data are very beneficial for improvements of simulators and comparison of simulators and results. Furthermore, they can be used for comparative studies, e.g. with measured natural rainfall spectra. A broad range of rainfall data was measured (e.g. intensity: 37-360mmh-1; Christiansen Coefficient for spatial rainfall distribution: 61-98{\%}; median volumetric drop diameter: 0.375-6.5mm; mean kinetic energy expenditure: 25-1322Jm-2h-1; mean kinetic energy per unit area and unit depth of rainfall: 0.77-50Jm-2mm-1). Similarities among the simulators could be found e.g. concerning drop size distributions (maximum drop numbers are reached within the smallest drop classes {\textless}1mm) and low fall velocities of bigger drops due to a general physical restriction. The comparison represents a good data-base for improvements and provides a consistent picture of the different parameters of the simulators that were tested. ?? 2013 Elsevier B.V.}, author = {Georgieva, Ivelina and Aquino, Ad{\'{e}}lia J.A. and Plasser, Felix and Trendafilova, Natasha and K{\"{o}}hn, Andreas and Lischka, Hans}, doi = {10.1021/acs.jpca.5b03282}, isbn = {0341-8162}, issn = {15205215}, journal = {J. Phys. Chem. A}, number = 24, pages = {6232–6243}, pmid = {25989536}, title = {{Intramolecular Charge-Transfer Excited-State Processes in 4-(N, N -Dimethylamino)benzonitrile: The Role of Twisting and the πσ∗ State}}, url = {http://dx.doi.org/10.1021/acs.jpca.5b03282}, volume = 119, year = 2015 }
  Link
  http://dx.doi.org/10.1021/acs.jpca.5b03282
4. Lin, C. H., Pursley, D., Klein, J. E. M. N., Teske, J., Allen, J. A., Rami, F., Köhn, A., & Plietker, B. “Non-decarbonylative photochemical versus thermal activation of Bu₄N［Fe(CO)₃(NO)］ - the Fe-catalyzed Cloke-Wilson rearrangement of vinyl and arylcyclopropanes.” Chem. Sci. 6, 7034–7043 (2015)
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  @article{AK57, abstract = {{\textless}p{\textgreater}TBA［Fe］ catalyzes the rearrangement of vinyl and arylcyclopropanes under thermal or photochemical conditions.{\textless}/p{\textgreater}}, author = {Lin, Che Hung and Pursley, Dominik and Klein, Johannes E M N and Teske, Johannes and Allen, Jennifer A. and Rami, Fabian and K{\"{o}}hn, Andreas and Plietker, Bernd}, doi = {10.1039/c5sc02342d}, issn = {20416539}, journal = {Chem. Sci.}, number = 12, pages = {7034–7043}, title = {{Non-decarbonylative photochemical versus thermal activation of Bu₄N［Fe(CO)₃(NO)］ - the Fe-catalyzed Cloke-Wilson rearrangement of vinyl and arylcyclopropanes}}, url = {http://dx.doi.org/10.1039/c5sc02342d}, volume = 6, year = 2015 }
  Link
  http://dx.doi.org/10.1039/c5sc02342d
5. Heidt, T., Baro, A., Köhn, A., & Laschat, S. “Synthesis of Cembranoid Analogues through Ring-Closing Metathesis of Terpenoid Precursors: A Challenge Regarding Ring-Size Selectivity.” Chem. - A Eur. J. 21, 12396–12404 (2015)
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  @article{AK55, abstract = {Together with 106 farmers who started growing Jatropha (Jatropha curcas$\backslash$nL.) in 20042006, this research sought to increase the knowledge around$\backslash$nthe real-life experience of Jatropha farming in the southern India$\backslash$nstates of Tamil Nadu and Andhra Pradesh. Launched as an alternative for$\backslash$ndiesel in India, Jatropha has been promoted as a non-edible plant that$\backslash$ncould grow on poor soils, yield oil-rich seeds for production of$\backslash$nbio-diesel, and not compete directly with food production. Through$\backslash$ninterviews with the farmers, information was gathered regarding their$\backslash$nsocio-economic situation, the implementation and performance of their$\backslash$nJatropha plantations, and their reasons for continuing or discontinuing$\backslash$nJatropha cultivation. Results reveal that 82{\%} of the farmers had$\backslash$nsubstituted former cropland for their Jatropha cultivation. By 2010,$\backslash$n85{\%} (n = 90) of the farmers who cultivated Jatropha in 2004 had$\backslash$nstopped. Cultivating the crop did not give the economic returns the$\backslash$nfarmers anticipated, mainly due to a lack of information about the crop$\backslash$nand its maintenance during cultivation and due to water scarcity. A$\backslash$nmajority of the farmers irrigated and applied fertilizer, and even$\backslash$npesticides. Many problems experienced by the farmers were due to limited$\backslash$nknowledge about cultivating Jatropha caused by poor planning and$\backslash$nimplementation of the national Jatropha program. Extension services,$\backslash$nsubsidies, and other support were not provided as promised. The farmers$\backslash$nwho continued cultivation had means of income other than Jatropha and$\backslash$nheld hopes of a future Jatropha market. The lack of market structures,$\backslash$nsuch as purchase agreements and buyers, as well as a low retail price$\backslash$nfor the seeds, were frequently stated as barriers to Jatropha$\backslash$ncultivation. For Jatropha biodiesel to perform well, efforts are needed$\backslash$nto improve yield levels and stability through genetic improvements and$\backslash$ndrought tolerance, as well as agriculture extension services to support$\backslash$nadoption of the crop. Government programs will -probably be more$\backslash$neffective if implementing biodiesel production is conjoined with$\backslash$nstimulating the demand for Jatropha biodiesel. To avoid food-biofuel$\backslash$ncompetition, additional measures may be needed such as land-use$\backslash$nrestrictions for Jatropha producers and taxes on biofuels or biofuel$\backslash$nfeedstocks to improve the competitiveness of the food sector compared to$\backslash$nthe bioenergy sector. (c) 2012 Society of Chemical Industry and John$\backslash$nWiley {\&} Sons, Ltd}, archiveprefix = {arXiv}, arxivid = {bk-16a}, author = {Heidt, Tanja and Baro, Angelika and K{\"{o}}hn, Andreas and Laschat, Sabine}, doi = {10.1002/chem.201502051}, eprint = {bk-16a}, isbn = {1932-104X}, issn = {15213765}, journal = {Chem. - A Eur. J.}, number = 35, pages = {12396–12404}, pmid = {26227568}, title = {{Synthesis of Cembranoid Analogues through Ring-Closing Metathesis of Terpenoid Precursors: A Challenge Regarding Ring-Size Selectivity}}, url = {http://dx.doi.org/10.1002/chem.201502051}, volume = 21, year = 2015 }
  Link
  http://dx.doi.org/10.1002/chem.201502051
2014
1. Diehl, F. P., Roos, C., Duymaz, A., Lunkenheimer, B., Köhn, A., & Basché, T. “Emergence of coherence through variation of intermolecular distances in a series of molecular dimers.” J. Phys. Chem. Lett. 5, 262–269 (2014)
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  @article{AK49, abstract = {Quantum coherences between electronically excited molecules are a signature of entanglement and play an important role for energy transport in molecular assemblies. Here we monitor and analyze for a homologous series of molecular dimers embedded in a solid host the emergence of coherence with decreasing intermolecular distance by single-molecule spectroscopy and quantum chemistry. Coherent signatures appear as an enhancement of the purely electronic transitions in the dimers which is reflected by changes of fluorescence spectra and lifetimes. Effects that destroy the coherence are the coupling to the surroundings and to vibrational excitations. Complementary information is provided by excitation spectra from which the electronic coupling strengths were extracted and found to be in good agreement with calculated values. By revealing various signatures of intermolecular coherence, our results pave the way for the rational design of molecular systems with entangled states.}, author = {Diehl, Florian P. and Roos, Claudia and Duymaz, Adile and Lunkenheimer, Bernd and K{\"{o}}hn, Andreas and Basch{\'{e}}, Thomas}, doi = {10.1021/jz402512g}, isbn = {1948-7185}, issn = {19487185}, journal = {J. Phys. Chem. Lett.}, number = 2, pages = {262–269}, title = {{Emergence of coherence through variation of intermolecular distances in a series of molecular dimers}}, url = {http://dx.doi.org/10.1021/jz402512g}, volume = 5, year = 2014 }
  Link
  http://dx.doi.org/10.1021/jz402512g
2. Samanta, P. K., Mukherjee, D., Hanauer, M., & Köhn, A. “Excited states with internally contracted multireference coupled-cluster linear response theory.” J. Chem. Phys. 140, 134108 (2014)
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  @article{AK50, abstract = {In this paper, the linear response (LR) theory for the variant of internally contracted multireference coupled cluster (ic-MRCC) theory described by Hanauer and K{\"{o}}hn ［J. Chem. Phys.134, 204211 (2011)］ has been formulated and implemented for the computation of the excitation energies relative to a ground state of pronounced multireference character. We find that straightforward application of the linear-response formalism to the time-averaged ic-MRCC Lagrangian leads to unphysical second-order poles. However, the coupling matrix elements that cause this behavior are shown to be negligible whenever the internally contracted approximation as such is justified. Hence, for the numerical implementation of the method, we adopt a Tamm-Dancoff-type approximation and neglect these couplings. This approximation is also consistent with an equation-of-motion based derivation, which neglects these couplings right from the start. We have implemented the linear-response approach in the ic-MRCC singles-and-doubles framework and applied our method to calculate excitation energies for a number of molecules ranging from CH₂ to p-benzyne and conjugated polyenes (up to octatetraene). The computed excitation energies are found to be very accurate, even for the notoriously difficult case of doubly excited states. The ic-MRCC-LR theory is also applicable to systems with open-shell ground-state wavefunctions and is by construction not biased towards a particular reference determinant. We have also compared the linear-response approach to the computation of energy differences by direct state-specific ic-MRCC calculations. We finally compare to Mk-MRCC-LR theory for which spurious roots have been reported ［T.-C. Jagau and J. Gauss, J. Chem. Phys.137, 044116 (2012)］, being due to the use of sufficiency conditions to solve the Mk-MRCC equations. No such problem is present in ic-MRCC-LR theory.}, author = {Samanta, Pradipta Kumar and Mukherjee, Debashis and Hanauer, Matthias and K{\"{o}}hn, Andreas}, doi = {10.1063/1.4869719}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 13, pages = 134108, title = {{Excited states with internally contracted multireference coupled-cluster linear response theory}}, url = {http://dx.doi.org/10.1063/1.4869719}, volume = 140, year = 2014 }
  Link
  http://dx.doi.org/10.1063/1.4869719
3. Aquino, A. A. J., Borges, I., Nieman, R., Köhn, A., & Lischka, H. “Intermolecular interactions and charge transfer transitions in aromatic hydrocarbon-tetracyanoethylene complexes.” Phys. Chem. Chem. Phys. 16, 20586–20597 (2014)
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  @article{AK51, abstract = {{\textless}p{\textgreater}ADC(2) calculations accurately describe charge transfer transitions in complexes of the tetracyanoethylene electron acceptor and three distinct aromatic donors.{\textless}/p{\textgreater}}, author = {Aquino, Ad{\'{e}}lia A.J. and Borges, Itamar and Nieman, Reed and K{\"{o}}hn, Andreas and Lischka, Hans}, doi = {10.1039/c4cp02900c}, isbn = {1463-9076}, issn = {14639076}, journal = {Phys. Chem. Chem. Phys.}, number = 38, pages = {20586–20597}, pmid = {25156236}, title = {{Intermolecular interactions and charge transfer transitions in aromatic hydrocarbon-tetracyanoethylene complexes}}, url = {http://dx.doi.org/10.1039/c4cp02900c}, volume = 16, year = 2014 }
  Link
  http://dx.doi.org/10.1039/c4cp02900c
4. Roos, C., Köhn, A., Gauss, J., & Diezemann, G. “The temperature dependence of vibronic lineshapes: Linear electron-phonon coupling.” J. Chem. Phys. 141, 154110 (2014)
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  @article{AK52, abstract = {We calculate the effect of a linear electron-phonon coupling on vibronic transitions of dye molecules of arbitrary complexity. With the assumption of known vibronic frequencies (for instance from quantum-chemical calculations), we give expressions for the absorption or emission lineshapes in a second-order cumulant expansion. We show that the results coincide with those obtained from generalized Redfield theory if one uses the time-local version of the theory and applies the secular approximation. Furthermore, the theory allows to go beyond the Huang-Rhys approximation and can be used to incorporate Dushinsky effects in the treatment of the temperature dependence of optical spectra. We consider both, a pure electron-phonon coupling independent of the molecular vibrations and a coupling bilinear in the molecular vibrational modes and the phonon coordinates. We discuss the behavior of the vibronic density of states for various models for the spectral density representing the coupling of the vibronic system to the harmonic bath. We recover some of the results that have been derived earlier for the spin-boson model and we show that the behavior of the spectral density at low frequencies determines the dominant features of the spectra. In case of the bilinear coupling between the molecular vibrations and the phonons we give analytical expressions for different spectral densities. The spectra are reminiscent of those obtained from the well known Brownian oscillator model and one finds a zero-phonon line and phonon-side bands located at vibrational frequencies of the dye. The intensity of the phonon-side bands diminishes with increasing vibrational frequencies and with decreasing coupling strength (Huang-Rhys factor). It vanishes completely in the Markovian limit where only a Lorentzian zero-phonon line is observed.}, author = {Roos, Claudia and K{\"{o}}hn, Andreas and Gauss, J{\"{u}}rgen and Diezemann, Gregor}, doi = {10.1063/1.4898081}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 15, pages = 154110, pmid = {25338884}, title = {{The temperature dependence of vibronic lineshapes: Linear electron-phonon coupling}}, url = {http://dx.doi.org/10.1063/1.4898081}, volume = 141, year = 2014 }
  Link
  http://dx.doi.org/10.1063/1.4898081
2013
1. Borges, I., Aquino, A. J. A., Köhn, A., Nieman, R., Hase, W. L., Chen, L. X., & Lischka, H. “Ab initio modeling of excitonic and Charge-transfer states in organic semiconductors: The PTB1/PCBM low band gap system.” J. Am. Chem. Soc. 135, 18252–18255 (2013)
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  @article{AK48, abstract = {A detailed quantum chemical simulation of the excitonic and charge-transfer (CT) states of a bulk heterojunction model containing poly(thieno［3,4-b］thiophene benzodithiophene) (PTB1)/［6,6］-phenyl-C61-butyric acid methyl ester (PCBM) is reported. The largest molecular model contains two stacked PTB1 trimer chains interacting with C60 positioned on top of and lateral to the (PTB1)3 stack. The calculations were performed using the algebraic diagrammatic construction method to second order (ADC(2)). One main result of the calculations is that the CT states are located below the bright inter-chain excitonic state, directly accessible via internal conversion processes. The other important aspects of the calculations are the formation of discrete bands of CT states originating from the lateral C60's and the importance of inter-chain charge delocalization for the stability of the CT states. A simple model for the charge separation step is also given, revealing the energetic feasibility of the overall photovoltaic process.}, author = {Borges, Itamar and Aquino, Ad{\'{e}}lia J.A. and K{\"{o}}hn, Andreas and Nieman, Reed and Hase, William L. and Chen, Lin X. and Lischka, Hans}, doi = {10.1021/ja4081925}, isbn = {0002-7863}, issn = {00027863}, journal = {J. Am. Chem. Soc.}, number = 49, pages = {18252–18255}, pmid = {24215627}, title = {{Ab initio modeling of excitonic and Charge-transfer states in organic semiconductors: The PTB1/PCBM low band gap system}}, url = {http://dx.doi.org/10.1021/ja4081925}, volume = 135, year = 2013 }
  Link
  http://dx.doi.org/10.1021/ja4081925
2. Köhn, A. “Complete basis set results in electron correlation methods using F12 theory.” Chem. Model. 10, 32–63 (2013)
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  @article{AK48B, author = {Köhn, Andreas}, doi = {10.1039/9781849737241-00032}, isbn = {9781849737241}, journal = {Chem. Model.}, pages = {32-63}, title = {{Complete basis set results in electron correlation methods using F12 theory}}, url = {http://dx.doi.org/10.1039/9781849737241-00032}, volume = 10, year = 2013 }
  Link
  http://dx.doi.org/10.1039/9781849737241-00032
3. Liu, W., Hanauer, M., & Köhn, A. “Explicitly correlated internally contracted multireference coupled-cluster singles and doubles theory: Ic-MRCCSD(F12*).” Chem. Phys. Lett. 565, 122–127 (2013)
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  @article{AK47, abstract = {An explicitly correlated ansatz employing Slater-type geminals and cusp conditions is developed for the internally contracted multireference coupled-cluster singles and doubles method. Only the most important geminal terms are retained in the spirit of earlier work for single-reference theory. Throughout all our test calculations, the new ic-MRCCSD(F12*) method improves the basis set convergence of many properties, e.g., spectroscopic constants or singlet-triplet splittings, with only little extra computational cost. If a perturbative correction for connected triples is included (the ic-MRCCSD(F12*)+(T) method), very accurate results can be obtained even with minimal active spaces. {\textcopyright} 2013 Elsevier B.V. All rights reserved.}, author = {Liu, Wenlan and Hanauer, Matthias and K{\"{o}}hn, Andreas}, doi = {10.1016/j.cplett.2012.12.052}, issn = {00092614}, journal = {Chem. Phys. Lett.}, pages = {122–127}, title = {{Explicitly correlated internally contracted multireference coupled-cluster singles and doubles theory: Ic-MRCCSD(F12*)}}, url = {http://dx.doi.org/10.1016/j.cplett.2012.12.052}, volume = 565, year = 2013 }
  Link
  http://dx.doi.org/10.1016/j.cplett.2012.12.052
4. Glaser, T., Beck, S., Lunkenheimer, B., Donhauser, D., Köhn, A., Kröger, M., & Pucci, A. “Infrared study of the MoO₃ doping efficiency in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP).” Org. Electron. Physics, Mater. Appl. 14, 575–583 (2013)
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  @article{AK45, abstract = {Electrochemical doping produces clear changes in the vibrational spectra of organic semiconductors as we show here for the system molybdenum oxide (MoO₃) doped into the charge transport material 4,4′-bis(N- carbazolyl)-1,1′-biphenyl (CBP). Based on density-functional theory (DFT) calculations of vibrational spectra, the new spectral features can be attributed to the CBP cation that forms as a result of electron transfer from CBP to MoO₃. The intensity of the new vibrational lines is a direct measure for the probability of charge transfer. MoO₃ agglomerating within the CBP matrix limits the active interface area between the two species. The appearance of a broad electronic transition in the infrared range indicates a new electronic structure at the interface compared to the individual components. The intensity of this electronic excitation serves as a measure for the interface area indicating a linear increase with MoO₃ concentration. Deposition onto cooled substrates results in smaller agglomerates, and thus yields a higher efficiency. {\textcopyright} 2012 Elsevier B.V. All rights reserved.}, author = {Glaser, Tobias and Beck, Sebastian and Lunkenheimer, Bernd and Donhauser, Daniela and K{\"{o}}hn, Andreas and Kr{\"{o}}ger, Michael and Pucci, Annemarie}, doi = {10.1016/j.orgel.2012.11.031}, isbn = {1566-1199}, issn = {15661199}, journal = {Org. Electron. physics, Mater. Appl.}, number = 2, pages = {575–583}, title = {{Infrared study of the MoO₃ doping efficiency in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP)}}, url = {http://dx.doi.org/10.1016/j.orgel.2012.11.031}, volume = 14, year = 2013 }
  Link
  http://dx.doi.org/10.1016/j.orgel.2012.11.031
5. Lunkenheimer, B., & Köhn, A. “Solvent effects on electronically excited states using the conductor-like screening model and the second-order correlated method ADC(2).” J. Chem. Theory Comput. 9, 977–994 (2013)
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  @article{AK46, abstract = {The conductor-like screening model (COSMO) is used to treat solvent effects on excited states within a correlated method based on the algebraic-diagrammatic construction through second-order ADC(2). The origin of solvent effects is revisited, and it is pointed out that two types of contributions have to be considered. One effect is due to the change of the solute's charge distribution after excitation, which triggers a reorganization of the solvent. Initially, only the electronic degrees of freedom adapt to the new charge distribution (nonequilibrium case); for sufficiently long-lived states, the reorientation of the solvent molecules contributes, as well (equilibrium case). The second effect is the coupling of the transition densities to the fast (purely electronic) response of the solvent molecules, which can be viewed as excitonic coupling between solute and solvent molecules. This interaction is also responsible for the screening of excitonic couplings between spatially separated chromophores. While most previous implementations of comparable continuum solvation models only include either of both effects, we argue that both contributions should be taken into account. Both effects can significantly influence the excitation energy and excited state properties of the solute, as exemplified for the π−π* and n−π* excitations of acrolein, and no a priori reason exists to neglect either. The implementation is also tested for the excitonic coupling of the ethene dimer where linear response contributions are indispensable for recovering the screening effects due to the solvent. Example applications to larger cases are provided, too. We discuss the excitonic coupling in a linked dyad consisting of two perylene-tetracarboxy-diimide chromophores, and the solvent effects on an intramolecular charge-transfer state of 4-(N,N-dimethylamino)benzonitrile}, author = {Lunkenheimer, Bernd and K{\"{o}}hn, Andreas}, doi = {10.1021/ct300763v}, issn = {15499626}, journal = {J. Chem. Theory Comput.}, number = 2, pages = {977–994}, pmid = {26588741}, title = {{Solvent effects on electronically excited states using the conductor-like screening model and the second-order correlated method ADC(2)}}, url = {http://dx.doi.org/10.1021/ct300763v}, volume = 9, year = 2013 }
  Link
  http://dx.doi.org/10.1021/ct300763v
6. Köhn, A., Hanauer, M., Mück, L. A., Jagau, T. C., & Gauss, J. “State-specific multireference coupled-cluster theory.” Wiley Interdiscip. Rev. Comput. Mol. Sci. 3, 176–197 (2013)
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  @article{AK44, abstract = {The multireference problem is considered one of the great challenges in coupled-cluster (CC) theory. Most recent developments are based on state-specific approaches, which focus on a single state and avoid some of the numerical problems of more general approaches. We review various state-of-the-art methods, including Mukherjee's state-specific multireference coupled-cluster (Mk-MRCC) theory, multireference Brillouin-Wigner coupled-cluster (MR-BWCC) theory, the MRexpT method, and internally contracted multireference coupled-cluster (ic-MRCC) theory. Related methods such as extended single-reference schemes ［e. g., the complete active space coupled-cluster (CASCC) theory］ and canonical transformation (CT) theory are covered as well. The comparison is done on the basis of formal arguments, implementation issues, and numerical results. Although a final and generally accepted multireference CC theory is still lacking, it is emphasized that recent developments render the new MRCC schemes useful tools for solving chemical problems. (C) 2012 John Wiley {\&} Sons, Ltd.}, author = {K{\"{o}}hn, Andreas and Hanauer, Matthias and M{\"{u}}ck, Leonie Anna and Jagau, Thomas Christian and Gauss, J{\"{u}}rgen}, doi = {10.1002/wcms.1120}, isbn = {1759-0876}, issn = {17590876}, journal = {Wiley Interdiscip. Rev. Comput. Mol. Sci.}, number = 2, pages = {176–197}, title = {{State-specific multireference coupled-cluster theory}}, url = {http://dx.doi.org/10.1002/wcms.1120}, volume = 3, year = 2013 }
  Link
  http://dx.doi.org/10.1002/wcms.1120
2012
1. Evangelista, F. A., Hanauer, M., Köhn, A., & Gauss, J. “A sequential transformation approach to the internally contracted multireference coupled cluster method.” J. Chem. Phys. 136, 204108 (2012)
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  @article{AK40, abstract = {The internally contracted multireference coupled cluster (ic-MRCC) approach is formulated using a new wave function ansatz based on a sequential transformation of the reference function (sqic-MRCC). This alternative wave function simplifies the formulation of computationally viable methods while preserving the accuracy of the ic-MRCC approach. The structure of the sqic-MRCC wave function allows folding the effect of the single excitations into a similarity-transformed Hamiltonian whose particle rank is equal to the one of the Hamiltonian. Consequently, we formulate an approximation to the sqic-MRCC method with singles and doubles (included respectively up to fourfold and twofold commutators, sqic-MRCCSD［2］) that contains all terms present in the corresponding single-reference coupled cluster scheme. Computations of the potential energy curves for the dissociation of BeH(2) show that the untruncated sqic-MRCCSD scheme yields results that are almost indistinguishable from the ordinary ic-MRCCSD method. The energy obtained from the computationally less expensive sqic-MRCCSD［2］ approximation is found to deviate from the full ic-MRCCSD method by less than 0.2 mE(h) for BeH(2), while, in the case of water, the harmonic vibrational frequencies of ozone, the singlet-triplet splitting of p-benzyne, and the dissociation curve of N(2), sqic-MRCCSD［2］ faithfully reproduces the results obtained via the ic-MRCCSD scheme truncated to two commutators. A formal proof is given of the equivalence of the ic-MRCC and sqic-MRCC methods with the internally contracted and full configuration interaction approaches.}, author = {Evangelista, Francesco A. and Hanauer, Matthias and K{\"{o}}hn, Andreas and Gauss, J{\"{u}}rgen}, doi = {10.1063/1.4718704}, isbn = {1089-7690 (Electronic)$\backslash$r0021-9606 (Linking)}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 20, pages = 204108, pmid = {22667541}, title = {{A sequential transformation approach to the internally contracted multireference coupled cluster method}}, url = {http://dx.doi.org/10.1063/1.4718704}, volume = 136, year = 2012 }
  Link
  http://dx.doi.org/10.1063/1.4718704
2. Pabst, M., Sundholm, D., & Köhn, A. “Ab initio studies of triplet-state properties for organic semiconductor molecules.” J. Phys. Chem. C 116, 15203–15217 (2012)
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  @article{AK41, abstract = {Triplet-triplet annihilation (TTA) leads to a reduced efficiency of organic light emitting diodes (OLEDs) at high current densities. Spacial confinement of the triplet excitons, which is mainly dependent on triplet energy differences, can reduce the TTA rate. Thus, a deliberate choice of the organic semiconductor materials with particular attention to their triplet energies can help to considerably increase the device efficiency. Organic solid-state lasers are on the other hand efficiently quenched by singlet-triplet annihilation (STA), which is closely related to the triplet-triplet absorption of the organic semiconductors. To establish a useful set of parameters related to the processes in organic semiconducting devices, we provide theoretical estimates for the triplet energy of 31 organic semiconductor molecules using state-of-the art ab initio quantum chemical methods. For a subset of 22 molecules the triplet-triplet absorption spectra were calculated, as well. We also discuss related features like localizations of excitations to molecular fragments, driven by the structural changes of the molecules in the excited triplet state. The calculated excited-state properties can assist experimentalists and serve as input parameters in simulations of organic electronics.}, author = {Pabst, Mathias and Sundholm, Dage and K{\"{o}}hn, Andreas}, doi = {10.1021/jp302010a}, isbn = {0023-2149 (Print)$\backslash$r0023-2149 (Linking)}, issn = {19327447}, journal = {J. Phys. Chem. C}, number = 29, pages = {15203–15217}, pmid = {22734677}, title = {{Ab initio studies of triplet-state properties for organic semiconductor molecules}}, url = {http://dx.doi.org/10.1021/jp302010a}, volume = 116, year = 2012 }
  Link
  http://dx.doi.org/10.1021/jp302010a
3. Hanauer, M., & Köhn, A. “Communication: Restoring full size extensivity in internally contracted multireference coupled cluster theory.” J. Chem. Phys. 137, 131103 (2012)
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  @article{AK43, abstract = {The reason for the lack of size extensivity in the valence space in current implementations of internally contracted multireference coupled cluster theories is the procedure used to eliminate redundant components from the cluster operator. We present a simple way to restore full size extensivity by performing this critical step in a basis of excitation operators that are normal ordered with respect to the multiconfigurational reference function.}, author = {Hanauer, Matthias and K{\"{o}}hn, Andreas}, doi = {10.1063/1.4757728}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 13, pages = 131103, pmid = {23039578}, title = {{Communication: Restoring full size extensivity in internally contracted multireference coupled cluster theory}}, url = {http://dx.doi.org/10.1063/1.4757728}, volume = 137, year = 2012 }
  Link
  http://dx.doi.org/10.1063/1.4757728
4. Hättig, C., Klopper, W., Köhn, A., & Tew, D. P. “Explicitly correlated electrons in molecules.” Chem. Rev. 112, 4–74 (2012)
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  @article{AK38, abstract = {The data are reviewed on the population structure and evolutionary dynamics of the nodule bacteria (rhizobia) which are among the most intensively studied microorganisms. High level of the population polymorphism was demonstrated for the rhizobia populations using the enzyme electrophoresis (MLEE profiles). The average value of Nei's coefficient of heterogeneity (H = 1 - Sigma (2)(Pi) ［n /(n - 1)］) were: 0,590 for rhizobia (Rhizobium, Bradyrhizobium), 0,368 for enterobacteria (Escherichia, Salmonella, Shigella) and 0,452 for pathogenic bacteria (Bordetella, Borrelia, Eryispelotrix, Haemophilus, Heliobacter, Listeria, Mycobacterium, Neisseria, Staphylococcus) populations. In spite of being devoid of the effective systems for the gene conjugative transfer, many rhizobia populations possess an essentially panmictic structure. However, the enterobacteria populations in which the gene transfer may be facilitated due to the conjugative F- and R-factors, usually display the clonal population structure. The legume host plant is proved to be a key factor that determines the high levels of polymorphism and of panmixia as well as high evolutionary rates of the symbiotic bacteria populations. The host may ensure: a) an increase in mutation and gene transfer frequencies; b) stimulation of the competitive (selective) processes in both symbiotic and free-living rhizobia populations. A "cyclic" model of the rhyzobia microevolution is presented which allows to assess the inputs the interstrain competition for the saprophytic growth and for the host nodulation into evolution of a plant-associated rhizobia population. The nodulation competitiveness in the rhizobia populations is responsible for the frequency-dependent selection of the rare genotypes which may arise in the soil bacterial communities as a result of the transfer of symbiotic (sym) genes from virulent rhizobia strains to either avirulent rhizobia or to the other (saprophytic, phytopathogenic) bacteria. Therefore, the nodulation competitiveness may ensure: a) panmictic structure of the natural rhizobia populations; b) high taxonomic diversity of rhizobia which was apparently caused by a broad sym gene expansion in the soil bacterial communities. The kin selection models are presented which explain evolution of the "altruistic" (essential for the host plant, but not for the bacteria themselves) symbiotic traits (e.g., the ability for symbiotic nitrogen fixation and for differentiation into non-viable bacteroids) in the rhizobia populations. These models are based on preferential multiplication of the nitrogen-fixing clones either in planta (due to an elevated supply of the nitrogen-fixing nodules with photosynthates) or ex planta (due to a release of the rhizopines from the nitrogen-fixing nodules). Speaking generally, interactions with the host plants provide a range of mechanisms increasing a genetic heterogeneity and an evolutionary potential in the associated rhizobia populations.}, author = {H{\"{a}}ttig, Christof and Klopper, Wim and K{\"{o}}hn, Andreas and Tew, David P.}, doi = {10.1021/cr200168z}, isbn = {0009-2665}, issn = {00092665}, journal = {Chem. Rev.}, number = 1, pages = {4–74}, pmid = {22206503}, title = {{Explicitly correlated electrons in molecules}}, url = {http://dx.doi.org/10.1021/cr200168z}, volume = 112, year = 2012 }
  Link
  http://dx.doi.org/10.1021/cr200168z
5. Hanauer, M., & Köhn, A. “Meaning and magnitude of the reduced density matrix cumulants.” Chem. Phys. 401, 50–61 (2012)
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  @article{AK37, abstract = {Within the framework of a generalized normal ordering (GNO), invented by Mukherjee ［1］, the reduced density matrix cumulants of the (multiconfigurational) reference wave function play a central role, as they arise directly from the contraction rules. The extended Wick theorem allows contractions of an arbitrary number of active annihilators and creators through a cumulant of corresponding rank. Because the cumulant rank truncates naturally only at the number of active spin orbitals, practical applications of the GNO concept seem to rely on a fast convergence of the cumulant series, allowing one to neglect cumulants with high rank. By computing cumulant norms for selected systems (and up to rank 16), we demonstrate that the cumulants decay approximately exponentially with increasing rank for single reference cases, while the convergence is generally slower for multireference cases. When strong left-right correlation is present as in the singlet state of a dissociated N2molecule, even the cumulant with maximum rank, $\lambda$12≈ -1.5 for a CAS(6, 6), is not negligible per se. Besides reporting numerical results, the authors reformulate the theory of reduced density matrices and their cumulants using a notation that is particularly easy to handle, highlighting the close connection to conventional statistics. From this statistical approach, a simple interpretation of reduced density matrices and cumulants follows, according to which an n-body cumulant is a measure for the correlation between the occupation numbers of n spin orbitals. This interpretation is also valid for cumulants with ranks exceeding the number of electrons in the system. {\textcopyright} 2011 Elsevier B.V. All rights reserved.}, author = {Hanauer, Matthias and K{\"{o}}hn, Andreas}, doi = {10.1016/j.chemphys.2011.09.024}, issn = {03010104}, journal = {Chem. Phys.}, pages = {50–61}, title = {{Meaning and magnitude of the reduced density matrix cumulants}}, url = {http://dx.doi.org/10.1016/j.chemphys.2011.09.024}, volume = 401, year = 2012 }
  Link
  http://dx.doi.org/10.1016/j.chemphys.2011.09.024
6. Hanauer, M., & Köhn, A. “Perturbative treatment of triple excitations in internally contracted multireference coupled cluster theory.” J. Chem. Phys. 136, 204107 (2012)
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  @article{AK39, abstract = {Internally contracted multireference coupled cluster (ic-MRCC) methods with perturbative treatment of triple excitations are formulated based on Dyall's definition of a zeroth-order Hamiltonian. The iterative models ic-MRCCSDT-1, ic-MRCC3, and their variants ic-MRCCSD(T), ic-MRCC(3) which determine the energy correction from triples by a non-iterative step are consistent in the single-reference limit with CCSDT-1a, CC3, CCSD(T), and CC(3), respectively. Numerical tests on the potential energy surfaces of BeH(2), H(2)O, and N(2) as well as on the structure and harmonic vibrational frequencies of the ozone molecule show that these methods account very well for higher order correlation effects. The ic-MRCCSD(T) method is further applied to the geometry optimization and harmonic frequencies of the symmetric vibrational modes of the binuclear transition metal oxide Ni(2)O(2), to the singlet-triplet splittings of o-, m-, and p-benzyne and to a ring-opening reaction of an azirine compound with the molecular formula C(6)H(7)NO. The size of the active spaces used in this study ranges from CAS(2,2) to CAS(8,8). Comparisons of results based on differently sized active spaces indicate that the ic-MRCCSD(T) method provides a highly accurate and efficient treatment of both static and dynamic electron correlation in connection with minimal active spaces.}, author = {Hanauer, Matthias and K{\"{o}}hn, Andreas}, doi = {10.1063/1.4718700}, isbn = {1089-7690 (Electronic)$\backslash$r0021-9606 (Linking)}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 20, pages = 204107, pmid = {22667540}, title = {{Perturbative treatment of triple excitations in internally contracted multireference coupled cluster theory}}, url = {http://dx.doi.org/10.1063/1.4718700}, volume = 136, year = 2012 }
  Link
  http://dx.doi.org/10.1063/1.4718700
7. Ambrosek, D., Köhn, A., Schulze, J., & Kühn, O. “Quantum chemical parametrization and spectroscopic characterization of the frenkel exciton hamiltonian for a J-aggregate forming perylene bisimide dye.” J. Phys. Chem. A 116, 11451–11458 (2012)
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  @article{AK42, abstract = {Quantum chemical and quantum dynamical calculations are performed for a bay-substituted perylene bisimide dye up to its hexameric aggregate. The aggregate structure is determined by employing the self-consistent charge density functional tight-binding (SCC-DFTB) approach including dispersion corrections. It is characterized by a stabilization via two chains of hydrogen bonds facilitated by amide functionalities. Focusing on the central embedded dimer, the Coulomb coupling for this J-aggregate is determined by means of the time-dependent density functional theory (TDDFT) to be -514 cm(-1). Exciton vibrational coupling is treated within the shifted oscillator model from which five strongly coupled modes per monomer are selected for inclusion into a minimal dynamic model. Performing wave packet propagations for a model employing up to 7 electronic states and 30 vibrational modes using the multiconfiguration time-dependent Hartree method, aggregate absorption spectra are obtained and compared to experiment.}, author = {Ambrosek, D. and K{\"{o}}hn, A. and Schulze, J. and K{\"{u}}hn, O.}, doi = {10.1021/jp3069706}, isbn = {0922-1425}, issn = {10895639}, journal = {J. Phys. Chem. A}, number = 46, pages = {11451–11458}, pmid = {22946964}, title = {{Quantum chemical parametrization and spectroscopic characterization of the frenkel exciton hamiltonian for a J-aggregate forming perylene bisimide dye}}, url = {http://dx.doi.org/10.1021/jp3069706}, volume = 116, year = 2012 }
  Link
  http://dx.doi.org/10.1021/jp3069706
2011
1. Hanauer, M., & Köhn, A. “Pilot applications of internally contracted multireference coupled cluster theory, and how to choose the cluster operator properly.” J. Chem. Phys. 134, 204111 (2011)
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  @article{AK36, abstract = {The internally contracted multireference coupled$\backslash$ncluster (icMRCC) method allows a highly accurate$\backslash$ndescription of both static and dynamic correlation$\backslash$nwith a computational scaling similar to single$\backslash$nreference coupled cluster theory. The authors show$\backslash$nthat the method can lose its orbital invariance and$\backslash$nsize consistency when no special care is taken in$\backslash$nthe elimination of redundant excitations. Using the$\backslash$nBeH₂ model system, four schemes are compared which$\backslash$ndiffer in their treatment of linear dependencies$\backslash$nbetween excitations of different rank (such as$\backslash$nbetween singles and doubles). While the energy$\backslash$ncurves agree within tens of μEh when truncating$\backslash$nthe cluster operator at double excitations$\backslash$n(icMRCCSD), inclusion of triple excitations$\backslash$n(icMRCCSDT) leads to significant differences of more$\backslash$nthan 1 mEh. One scheme clearly yields the best$\backslash$nresults, while the others even turn out to be not$\backslash$nsize consistent. The former procedure uses genuine$\backslash$nsingle and double excitations and discards those$\backslash$nlinear combinations of (spectator) double and triple$\backslash$nexcitations which have the same effect on the$\backslash$nreference function. With this approach, the$\backslash$nequilibrium structure and harmonic vibrational$\backslash$nfrequencies of ozone obtained with icMRCCSDT are in$\backslash$nexcellent agreement with CCSDTQ. The authors further$\backslash$napply icMRCC methods to potential energy surfaces of$\backslash$nHF, LiF, N2, and to the singlet-triplet splitting of$\backslash$nbenzynes. In particular, the latter calculations$\backslash$nhave been made possible by implementing the method$\backslash$nwith the proper formal scaling using automated$\backslash$ntechniques.}, author = {Hanauer, Matthias and K{\"{o}}hn, Andreas}, doi = {10.1063/1.3592786}, isbn = {1089-7690 (Electronic)$\backslash$r0021-9606 (Linking)}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 20, pages = 204111, pmid = {21639428}, title = {{Pilot applications of internally contracted multireference coupled cluster theory, and how to choose the cluster operator properly}}, url = {http://dx.doi.org/10.1063/1.3592786}, volume = 134, year = 2011 }
  Link
  http://dx.doi.org/10.1063/1.3592786
2. Pabst, M., Lunkenheimer, B., & Köhn, A. “The triplet excimer of naphthalene: A model system for triplet-triplet interactions and its spectral properties.” J. Phys. Chem. C 115, 8335–8344 (2011)
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  @article{AK35, abstract = {Basic concepts of triplet excimer formation and triplet−triplet interactions between molecules with conjugated π-systems are investigated by means of ab initio quantum chemical calculations, employing the second-order coupled-cluster method CC2 and the second-order propagator method ADC(2). The naphthalene dimer turns out to be a very fruitful model system for which weak and strong electronic coupling can be identified depending on the mutual arrangement of the monomer moieties. From geometry optimizations in the excited state, we determine binding energies, including solvent effects, and transient absorption spectra. The most stable T1 conformation turns out to be a face-to-face arrangement with a rather short intermolecular distance. Its stabilization can be rationalized by a very strong electronic coupling due to a maximum overlap of the π-systems, which allows a strong admixture of charge-transfer configurations. Conformations without π−π-overlap show only a weak coupling and are less stable. The pred...}, author = {Pabst, Mathias and Lunkenheimer, Bernd and K{\"{o}}hn, Andreas}, doi = {10.1021/jp200818q}, isbn = {1932-7447}, issn = {19327447}, journal = {J. Phys. Chem. C}, number = 16, pages = {8335–8344}, pmid = {289697400063}, title = {{The triplet excimer of naphthalene: A model system for triplet-triplet interactions and its spectral properties}}, url = {http://dx.doi.org/10.1021/jp200818q}, volume = 115, year = 2011 }
  Link
  http://dx.doi.org/10.1021/jp200818q
2010
1. Pabst, M., Köhn, A., Gauss, J., & Stanton, J. F. “A worrisome failure of the CC2 coupled-cluster method when applied to ozone.” Chem. Phys. Lett. 495, 135–140 (2010)
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  @article{AK31, abstract = {The approximate coupled-cluster singles and doubles model CC2 is widely used for calculations of excited states in large molecules. We demonstrate a surprising failure of the CC2 geometry optimization of ozone, whereby the CC2 method predicts a barrierless, exothermic, and symmetric dissociation to three oxygen atoms. This is particularly astonishing since both second-order Møller-Plesset perturbation theory and coupled-cluster singles and doubles give reasonable equilibrium structures. We find that ［［H,T1］,T1］ leads to an unbalanced treatment of T1and T2in the CC2 equations. Results presented here suggest that considerable caution should be exercised when applying the CC2 model to systems with singlet biradical character. {\textcopyright} 2010 Elsevier B.V. All rights reserved.}, author = {Pabst, Mathias and K{\"{o}}hn, Andreas and Gauss, J{\"{u}}rgen and Stanton, John F.}, doi = {10.1016/j.cplett.2010.06.023}, isbn = {00092614}, issn = {00092614}, journal = {Chem. Phys. Lett.}, number = {1-3}, pages = {135–140}, title = {{A worrisome failure of the CC2 coupled-cluster method when applied to ozone}}, url = {http://dx.doi.org/10.1016/j.cplett.2010.06.023}, volume = 495, year = 2010 }
  Link
  http://dx.doi.org/10.1016/j.cplett.2010.06.023
2. Diehl, F. P., Roos, C., Jankowiak, H. C., Berger, R., Köhn, A., Diezemann, G., & Basché, T. “Combined experimental and theoretical study of the vibronic spectra of perylenecarboximides.” J. Phys. Chem. B 114, 1638–1647 (2010)
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  @article{AK29, abstract = {Absorption and emission spectra of perylene-3,4-dicarboximide (PMI) and perylene-3,4,9,10-tetracarboxdiimide (PDI) derivatives embedded in a thin polymer film were measured by room-temperature bulk and low-temperature single-molecule spectroscopy. In contrast to bulk line narrowing spectra, the low-temperature single-molecule data allowed to unambiguously resolve the vibrational fine structure of the emission spectra. Additionally, the emission spectra were calculated by quantum chemical methods within the Franck?Condon approximation for various N-substituted derivatives of PMI and PDI. The experimental as well as calculated emission spectra are dominated by two spectral regions of high vibronic activity, a band system ranging from the 0?0 transition (at ?E0?0) down to 600 cm?1 below ?E0?0 and a band system between approximately 1250 and 1700 cm?1 below ?E0?0. Apart from the wavenumber region close to ?E0?0 (down to 100 cm?1 below ?E0?0), good agreement is found between the calculated and experimental spectra, allowing a clear-cut assignment of the dominant vibrational modes. There are, however, discrepancies in the intensities in particular for low-frequency vibrational modes. These differences between theory and experiment are tentatively attributed to linear electron?phonon coupling which is completely neglected in the calculations and hindered internal rotation that is not properly accounted for in the harmonic approximation. Furthermore, in the experimental spectra, at the bulk as well as the single-molecule level, significant differences between PMI and PDI are observed which are attributed to stronger interactions with the matrix environment in the case of PMI due to the permanent electric dipole moment of that molecule. Absorption and emission spectra of perylene-3,4-dicarboximide (PMI) and perylene-3,4,9,10-tetracarboxdiimide (PDI) derivatives embedded in a thin polymer film were measured by room-temperature bulk and low-temperature single-molecule spectroscopy. In contrast to bulk line narrowing spectra, the low-temperature single-molecule data allowed to unambiguously resolve the vibrational fine structure of the emission spectra. Additionally, the emission spectra were calculated by quantum chemical methods within the Franck?Condon approximation for various N-substituted derivatives of PMI and PDI. The experimental as well as calculated emission spectra are dominated by two spectral regions of high vibronic activity, a band system ranging from the 0?0 transition (at ?E0?0) down to 600 cm?1 below ?E0?0 and a band system between approximately 1250 and 1700 cm?1 below ?E0?0. Apart from the wavenumber region close to ?E0?0 (down to 100 cm?1 below ?E0?0), good agreement is found between the calculated and experimental spectra, allowing a clear-cut assignment of the dominant vibrational modes. There are, however, discrepancies in the intensities in particular for low-frequency vibrational modes. These differences between theory and experiment are tentatively attributed to linear electron?phonon coupling which is completely neglected in the calculations and hindered internal rotation that is not properly accounted for in the harmonic approximation. Furthermore, in the experimental spectra, at the bulk as well as the single-molecule level, significant differences between PMI and PDI are observed which are attributed to stronger interactions with the matrix environment in the case of PMI due to the permanent electric dipole moment of that molecule.}, author = {Diehl, Florian P. and Roos, Claudia and Jankowiak, Hans Christian and Berger, Robert and K{\"{o}}hn, Andreas and Diezemann, Gregor and Basch{\'{e}}, Thomas}, doi = {10.1021/jp909862x}, isbn = {1520-6106}, issn = {15206106}, journal = {J. Phys. Chem. B}, number = 4, pages = {1638–1647}, pmid = {20058882}, title = {{Combined experimental and theoretical study of the vibronic spectra of perylenecarboximides}}, url = {http://dx.doi.org/10.1021/jp909862x}, volume = 114, year = 2010 }
  Link
  http://dx.doi.org/10.1021/jp909862x
3. Hättig, C., Tew, D. P., & Köhn, A. “Communications: Accurate and efficient approximations to explicitly correlated coupled-cluster singles and doubles, CCSD-F12.” J. Chem. Phys. 132, 231102 (2010)
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  @article{AK32, abstract = {We propose a novel explicitly correlated$\backslash$ncoupled-cluster singles and doubles method$\backslash$nCCSD(F12*), which retains the accuracy of$\backslash$nCCSD-F12 while the computational costs are only$\backslash$ninsignificantly larger than those for a conventional$\backslash$nCCSD calculation.}, author = {H{\"{a}}ttig, Christof and Tew, David P. and K{\"{o}}hn, Andreas}, doi = {10.1063/1.3442368}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 23, pages = 231102, pmid = {20572681}, title = {{Communications: Accurate and efficient approximations to explicitly correlated coupled-cluster singles and doubles, CCSD-F12}}, url = {http://dx.doi.org/10.1063/1.3442368}, volume = 132, year = 2010 }
  Link
  http://dx.doi.org/10.1063/1.3442368
4. Pabst, M., & Köhn, A. “Excited states of ［3.3］(4,4′)biphenylophane: The role of charge-transfer excitations in dimers with π-π interaction.” J. Phys. Chem. A 114, 1639–1649 (2010)
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  @article{AK28, abstract = {The singlet and triplet electronic excitation manifold of ［3.3］(4,4')biphenylophane (BPP), an intramolecular dimer, and 4,4'-dimethylbiphenyl (DMBP), the corresponding monomer, has been analyzed by employing the approximate coupled-cluster singles and doubles model (CC2). The calculated triplet-triplet and singlet-singlet transient absorption spectra show good agreement with recent experimental results. The calculations suggest a strong interaction of the two biphenyl moieties of BPP in the first singlet and triplet excited states due to the overlapping pi-electron systems, and Forster-Dexter theory for weak coupling cannot be applied. Both the first excited singlet and triplet states of BPP can be characterized as excimeric states with a strong admixture of charge-transfer contributions.}, author = {Pabst, Mathias and K{\"{o}}hn, Andreas}, doi = {10.1021/jp910551u}, issn = {10895639}, journal = {J. Phys. Chem. A}, number = 4, pages = {1639–1649}, pmid = {20055397}, title = {{Excited states of ［3.3］(4,4′)biphenylophane: The role of charge-transfer excitations in dimers with π-π interaction}}, url = {http://dx.doi.org/10.1021/jp910551u}, volume = 114, year = 2010 }
  Link
  http://dx.doi.org/10.1021/jp910551u
5. Köhn, A., & Tew, D. P. “Explicitly correlated coupled-cluster theory using cusp conditions. I. Perturbation analysis of coupled-cluster singles and doubles (CCSD-F12).” J. Chem. Phys. 133, 174117 (2010)
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  @article{AK33, abstract = {Geminal functions based on Slater-type correlation factors and fixed expansion coefficients, determined by cusp conditions, have in recent years been forwarded as an efficient and numerically stable method for introducing explicit electron correlation into coupled-cluster theory. In this work, we analyze the equations of explicitly correlated coupled-cluster singles and doubles (CCSD-F12) theory and introduce an ordering scheme based on perturbation theory which can be used to characterize and understand the various approximations found in the literature. Numerical results for a test set of 29 molecules support our analysis and give additional insight. In particular, our results help rationalize the success of the CCSD(F12) approximation which is based on a very systematic cancellation of the neglected, otherwise individually large third-order geminal-geminal coupling terms. Further approximations to CCSD(F12) can be introduced without sacrificing the accuracy if the entire set of third-order coupling terms between the conventional doubles cluster amplitudes and the geminal doubles amplitudes is retained, leading to the recently proposed CCSD［F12］ and CCSD(F12(∗)) models, which have negligible overhead compared to conventional CCSD calculations. Particularly, the importance of the ring-term type contribution is pointed out which may be used to improve on other existing approximations such as CCSD-F12b. For small basis sets, it might be advantageous to keep certain higher-order terms leading to CCSD-F12(∗), which, for the case of the SP ansatz, merely involves a noniterative correction to CCSD(F12(∗)).}, author = {K{\"{o}}hn, Andreas and Tew, David P.}, doi = {10.1063/1.3496372}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 17, pages = 174117, pmid = {21054016}, title = {{Explicitly correlated coupled-cluster theory using cusp conditions. I. Perturbation analysis of coupled-cluster singles and doubles (CCSD-F12)}}, url = {http://dx.doi.org/10.1063/1.3496372}, volume = 133, year = 2010 }
  Link
  http://dx.doi.org/10.1063/1.3496372
6. Köhn, A. “Explicitly correlated coupled-cluster theory using cusp conditions. II. Treatment of connected triple excitations.” J. Chem. Phys. 133, 174118 (2010)
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  @article{AK34, abstract = {The coupled-cluster singles and doubles method augmented with single Slater-type correlation factors (CCSD-F12) determined by the cusp conditions (also denoted as SP ansatz) yields results close to the basis set limit with only small overhead compared to conventional CCSD. Quantitative calculations on many-electron systems, however, require to include the effect of connected triple excitations at least. In this contribution, the recently proposed ［A. Köhn, J. Chem. Phys. 130, 131101 (2009)］ extended SP ansatz and its application to the noniterative triples correction CCSD(T) is reviewed. The approach allows to include explicit correlation into connected triple excitations without introducing additional unknown parameters. The explicit expressions are presented and analyzed, and possible simplifications to arrive at a computationally efficient scheme are suggested. Numerical tests based on an implementation obtained by an automated approach are presented. Using a partial wave expansion for the neon atom, we can show that the proposed ansatz indeed leads to the expected (L(max)+1)(-7) convergence of the noniterative triples correction, where L(max) is the maximum angular momentum in the orbital expansion. Further results are reported for a test set of 29 molecules, employing Peterson's F12-optimized basis sets. We find that the customary approach of using the conventional noniterative triples correction on top of a CCSD-F12 calculation leads to significant basis set errors. This, however, is not always directly visible for total CCSD(T) energies due to fortuitous error compensation. The new approach offers a thoroughly explicitly correlated CCSD(T)-F12 method with improved basis set convergence of the triples contributions to both total and relative energies.}, author = {K{\"{o}}hn, Andreas}, doi = {10.1063/1.3496373}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 17, pages = 174118, pmid = {21054017}, title = {{Explicitly correlated coupled-cluster theory using cusp conditions. II. Treatment of connected triple excitations}}, url = {http://dx.doi.org/10.1063/1.3496373}, volume = 133, year = 2010 }
  Link
  http://dx.doi.org/10.1063/1.3496373
7. Köhn, A., & Tew, D. P. “Towards the Hartree-Fock and coupled-cluster singles and doubles basis set limit: A study of various models that employ single excitations into a complementary auxiliary basis set.” J. Chem. Phys. 132, 24101 (2010)
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  @article{AK30, abstract = {In explicitly correlated coupled-cluster singles and doubles ［CCSD(F12)］ calculations, the basis set incompleteness error in the double excitations is reduced to such an extent that the error in the Hartree-Fock energy and the error in the single excitations become important. Using arguments from perturbation theory to systematically truncate the coupled-cluster singles and CCSD(F12) Lagrangians, a series of coupled-cluster models are proposed and studied that reduce these basis set incompleteness errors through additional single excitations into a complementary auxiliary basis. Convergence with model and size of complementary basis is rapid and there appears to be no need to go beyond second-order models. Our iterative second-order approach is a slight improvement over the existing noniterative approach, but its main advantage is that it is suitable for response theory.}, author = {K{\"{o}}hn, Andreas and Tew, David P.}, doi = {10.1063/1.3291040}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 2, pages = 24101, pmid = {20095657}, title = {{Towards the Hartree-Fock and coupled-cluster singles and doubles basis set limit: A study of various models that employ single excitations into a complementary auxiliary basis set}}, url = {http://dx.doi.org/10.1063/1.3291040}, volume = 132, year = 2010 }
  Link
  http://dx.doi.org/10.1063/1.3291040
2009
1. Köhn, A. “A modified ansatz for explicitly correlated coupled-cluster wave functions that is suitable for response theory.” J. Chem. Phys. 130, 104104 (2009)
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  @article{AK24, abstract = {A modified ansatz for explicitly correlated coupled-cluster wave functions with a single correlation factor is set forward. It is based on the fixed amplitude ansatz of Ten-no ［Chem. Phys. Lett. 398, 56 (2004)］ to which an extra term is added that allows for the explicitly correlated description of singly excited configurations. The new approach has been implemented for coupled-cluster singles and doubles with the aid of automated techniques. Numerical results are presented for vertical excitation energies, and ground and excited state equilibrium distances and harmonic frequencies of diatomics. The new approach is shown to provide a nearly unbiased description of ground and predominantly singly exited states, and the improvements seen for explicitly correlated treatments of ground states, as compared to conventional orbital expansions, carry over to excited states. In addition, a correction for contributions from one-electron terms-which are not improved by the correlation factor-is suggested, again with focus on applicability to a response formalism.}, author = {K{\"{o}}hn, Andreas}, doi = {10.1063/1.3079543}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 10, pages = 104104, pmid = {19292520}, title = {{A modified ansatz for explicitly correlated coupled-cluster wave functions that is suitable for response theory}}, url = {http://dx.doi.org/10.1063/1.3079543}, volume = 130, year = 2009 }
  Link
  http://dx.doi.org/10.1063/1.3079543
2. Köhn, A. “Explicitly correlated connected triple excitations in coupled-cluster theory.” J. Chem. Phys. 130, 131101 (2009)
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  @article{AK25, abstract = {A way to incorporate explicit electron correlation into connected triple excitations in coupled-cluster theory is proposed. The new ansatz is applied to the coupled-cluster singles and doubles model with noniterative triple excitations ［CCSD(T)］ and does not introduce any further sets of equations to be solved. A first implementation using automated generation and string-based evaluation of the explicit expressions is reported. The results demonstrate that the ansatz significantly enhances the basis set convergence of the noniterative triple excitation correction and thus improves upon previous approaches to explicitly correlated CCSD(T).}, author = {K{\"{o}}hn, Andreas}, doi = {10.1063/1.3116792}, isbn = {doi:10.1063/1.3116792}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 13, pages = 131101, pmid = {19355710}, title = {{Explicitly correlated connected triple excitations in coupled-cluster theory}}, url = {http://dx.doi.org/10.1063/1.3116792}, volume = 130, year = 2009 }
  Link
  http://dx.doi.org/10.1063/1.3116792
3. Sagvolden, E., Furche, F., & Köhn, A. “Förster energy transfer and davydov splittings in time-dependent density functional theory: lessons from 2-pyridone dimer.” J. Chem. Theory Comput. 5, 873–880 (2009)
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  @article{AK26, abstract = {The Davydov or exciton splitting of vertical excitation energies is commonly used to estimate the excitation energy transfer rate between chromophores. Here we investigate the S1?S2 Davydov splitting in 2-pyridone dimer as a function of the monomer separation, R. We assess the ability of various functionals to reproduce the Davydov splitting at finite R predicted by the approximate coupled cluster singles doubles method CC2. While semilocal functionals fail qualitatively because of spurious charge-transfer intruder states, global hybrids with a large fraction of exact exchange, such as BHandH-LYP, reproduce the CC2 splittings within few wavenumbers. We analyze our results by comparison to lowest-order intermolecular perturbation theory in the spirit of Fo?rster and Dexter. At equilibrium hydrogen bond distance, the Fo?rster?Dexter splittings are too small by up to a factor of 2.}, author = {Sagvolden, Espen and Furche, Filipp and K{\"{o}}hn, Andreas}, doi = {10.1021/ct800551g}, isbn = {1549-9618}, issn = {15499618}, journal = {J. Chem. Theory Comput.}, number = 4, pages = {873–880}, title = {{F{\"{o}}rster energy transfer and davydov splittings in time-dependent density functional theory: lessons from 2-pyridone dimer}}, url = {http://dx.doi.org/10.1021/ct800551g}, volume = 5, year = 2009 }
  Link
  http://dx.doi.org/10.1021/ct800551g
4. Hanauer, M., & Köhn, A. “Response properties with explicitly correlated coupled-cluster methods using a Slater-type correlation factor and cusp conditions.” J. Chem. Phys. 131, 124118 (2009)
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  @article{AK27, abstract = {The recently proposed extension of the explicitly correlated coupled-cluster ansatz using cusp conditions ［A. Kohn, J. Chem. Phys. 130, 104104 (2009)］ is tested for suitability in the calculation of response properties. For this purpose, static and dynamic electrical properties up to ESHG hyperpolarizabilities as well as optical rotations have been computed within the CCSD(F12) model. It is shown that effectively converged correlation contributions can reliably be obtained using augmented quadruple zeta basis sets already. The ansatz is optionally equipped with an extension capable of reducing the one-electron basis set error. A further simplification of the method specific Lagrangian aimed at reducing the computational effort has been tested and is shown to be uncritical. Furthermore, we examined the impact of conventional triple and quadruple excitations in explicitly correlated property calculations.}, author = {Hanauer, Matthias and K{\"{o}}hn, Andreas}, doi = {10.1063/1.3238237}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 12, pages = 124118, pmid = {19791863}, title = {{Response properties with explicitly correlated coupled-cluster methods using a Slater-type correlation factor and cusp conditions}}, url = {http://dx.doi.org/10.1063/1.3238237}, volume = 131, year = 2009 }
  Link
  http://dx.doi.org/10.1063/1.3238237
2008
1. Köhn, A., Richings, G. W., & Tew, D. P. “Implementation of the full explicitly correlated coupled-cluster singles and doubles model CCSD-F12 with optimally reduced auxiliary basis dependence.” J. Chem. Phys. 129, 201103 (2008)
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  @article{AK22, abstract = {An implementation of the full explicitly correlated coupled-cluster singles and doubles model CCSD-F12 using a single Slater-type geminal has been obtained with the aid of automated term generation and evaluation techniques. In contrast to a previously reported computer code ［T. Shiozaki et al., J. Chem. Phys. 129, 071101 (2008)］, our implementation features a reduced dependence on the auxiliary basis set due to the use of a reformulated evaluation of the so-called Z-intermediate rather than straight forward insertion of an auxiliary basis expansion, which allows an unambiguous comparison to more approximate CCSD-F12 models. First benchmark results for total correlation energies and reaction energies indicate an excellent performance of the much cheaper CCSD(F12) model.}, author = {K{\"{o}}hn, Andreas and Richings, Gareth W. and Tew, David P.}, doi = {10.1063/1.3028546}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 20, pages = 201103, pmid = {19045845}, title = {{Implementation of the full explicitly correlated coupled-cluster singles and doubles model CCSD-F12 with optimally reduced auxiliary basis dependence}}, url = {http://dx.doi.org/10.1063/1.3028546}, volume = 129, year = 2008 }
  Link
  http://dx.doi.org/10.1063/1.3028546
2. Pabst, M., & Köhn, A. “Implementation of transition moments between excited states in the approximate coupled-cluster singles and doubles model.” J. Chem. Phys. 129, 214101 (2008)
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  @article{AK23, abstract = {An implementation of transition moments between excited states for the approximate coupled-cluster singles and doubles model (CC2) using the resolution of the identity (RI) approximation is reported. The accuracy of the RI approximation is analyzed for a testset of 7 molecules and 76 transitions. The RI error is found to be very small for both transition moments and oscillator strengths. Furthermore, the performance of the CC2 model in comparison with coupled-cluster singles and doubles (CCSD) is studied for 40 transitions of the same testset, yielding deviations of about 12{\%} for the transition moments and 24{\%} for the oscillator strengths. In addition, for 13 transitions of the testset the behavior of the transition moments with respect to seven different basis sets (Dunnings xaug-cc-pVXZ, with x=0,1,2 for X=D,T and x=2 for X=5) is analyzed, showing a strong dependence on the degree of augmentation x and a rather small effect of the cardinal number X. First applications are presented for the triplet-triplet transition moments of benzene and polyacenes (naphthalene to pentacene), showing good agreement with experimental and theoretical results for transitions between single excitation dominated states. Somewhat problematic is the insufficient description of double-excitation dominated states by the CC2 model. As transitions to such states may be strongly allowed, unlike for excitations out of the ground state, important features of transient spectra may be missed. For triplet-triplet excitations the problem is less evident as the lowest doubly excited triplet states are expected at higher energies.}, author = {Pabst, Mathias and K{\"{o}}hn, Andreas}, doi = {10.1063/1.3023118}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 21, pages = 214101, pmid = {19063538}, title = {{Implementation of transition moments between excited states in the approximate coupled-cluster singles and doubles model}}, url = {http://dx.doi.org/10.1063/1.3023118}, volume = 129, year = 2008 }
  Link
  http://dx.doi.org/10.1063/1.3023118
3. Fückel, B., Köhn, A., Harding, M. E., Diezemann, G., Hinze, G., Basch́, T., & Gauss, J. “Theoretical investigation of electronic excitation energy transfer in bichromophoric assemblies.” J. Chem. Phys. 128, 74505 (2008)
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  @article{AK21, abstract = {Electronic excitation energy transfer (EET) rates in rylene diimide dyads are calculated using second-order approximate coupled-cluster theory and time-dependent density functional theory. We investigate the dependence of the EET rates on the interchromophoric distance and the relative orientation and show that Forster theory works quantitatively only for donor-acceptor separations larger than roughly 5 nm. For smaller distances the EET rates are over- or underestimated by Forster theory depending on the respective orientation of the transition dipole moments of the chromophores. In addition to the direct transfer rates we consider bridge-mediated transfer originating from oligophenylene units placed between the chromophores. We find that the polarizability of the bridge significantly enhances the effective interaction. We compare our calculations to single molecule experiments on two types of dyads and find reasonable agreement between theory and experiment.}, author = {F{\"{u}}ckel, Burkhard and K{\"{o}}hn, Andreas and Harding, Michael E. and Diezemann, Gregor and Hinze, Gerald and Basch́, Thomas and Gauss, J{\"{u}}rgen}, doi = {10.1063/1.2829531}, isbn = {0021-9606 (Print)$\backslash$r0021-9606 (Linking)}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 7, pages = 74505, pmid = {18298155}, title = {{Theoretical investigation of electronic excitation energy transfer in bichromophoric assemblies}}, url = {http://dx.doi.org/10.1063/1.2829531}, volume = 128, year = 2008 }
  Link
  http://dx.doi.org/10.1063/1.2829531
2007
1. Köhn, A., & Tajti, A. “Can coupled-cluster theory treat conical intersections?” J. Chem. Phys. 127, 44105 (2007)
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  @article{AK20, abstract = {Conical intersections between electronic states are of great importance for the understanding of radiationless ultrafast relaxation processes. In particular, accidental degeneracies of hypersurfaces, i.e., between states of the same symmetry, become increasingly relevant for larger molecular systems. Coupled-cluster theory, including both single and multireference based schemes, offers a size-extensive description of the electronic wave function, but it sacrifices the Hermitian character of the theory. In this contribution, we examine the consequences of anti-Hermitian contributions to the coupling matrix element between near-degenerate states such as linear dependent eigenvectors and complex eigenvalues. Numerical examples are given for conical intersections between two excited states calculated at the equation-of-motion coupled-cluster level which indeed show the predicted artifacts. A simple method is suggested which allows physically meaningful potential energy surfaces to be extracted from the otherwise ill-behaved results. This provides a perspective for obtaining potential energy surfaces near conical intersections at the coupled-cluster level.}, author = {K{\"{o}}hn, Andreas and Tajti, Attila}, doi = {10.1063/1.2755681}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 4, pages = 44105, pmid = {17672679}, title = {{Can coupled-cluster theory treat conical intersections?}}, url = {http://dx.doi.org/10.1063/1.2755681}, volume = 127, year = 2007 }
  Link
  http://dx.doi.org/10.1063/1.2755681
2006
1. Köhn, A., & Olsen, J. “Coupled-cluster with active space selected higher amplitudes: Performance of seminatural orbitals for ground and excited state calculations.” J. Chem. Phys. 125, 174110 (2006)
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  @article{AK17, abstract = {The active space approach for coupled-cluster models is generalized using the general active space concept and implemented in a string-based general coupled-cluster code. Particular attention is devoted to the choice of orbitals on which the subspace division is based. Seminatural orbitals are proposed for that purpose. These orbitals are obtained by diagonalizing only the hole-hole and particle-particle block of the one-electron density of a lower-order method. The seminatural orbitals are shown to be a good replacement for complete active space self-consistent field orbitals and avoid the ambiguities with respect to the reference determinant introduced by the latter orbitals. The seminatural orbitals also perform well in excited state calculations, including excited states with strong double excitation contributions, which usually are difficult to describe with standard coupled-cluster methods. A set of vertical excitation energies is obtained and benchmarked against full configuration interaction calculations, and alternative hierarchies of active space coupled-cluster models are proposed. As a simple application the spectroscopic constants of the C(2) B (1)Delta(g) and B(') (1)Sigma(g) (+) states are calculated using active space coupled-cluster methods and basis sets up to quadruple-zeta quality in connection with extrapolation and additivity schemes.}, author = {K{\"{o}}hn, Andreas and Olsen, Jeppe}, doi = {10.1063/1.2364491}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 17, pages = 174110, pmid = {17100432}, title = {{Coupled-cluster with active space selected higher amplitudes: Performance of seminatural orbitals for ground and excited state calculations}}, url = {http://dx.doi.org/10.1063/1.2364491}, volume = 125, year = 2006 }
  Link
  http://dx.doi.org/10.1063/1.2364491
2. Hättig, C., Hellweg, A., & Köhn, A. “Distributed memory parallel implementation of energies and gradients for second-order Møller-Plesset perturbation theory with the resolution-of-the-identity approximation.” Phys. Chem. Chem. Phys. 8, 1159–1169 (2006)
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  @article{AK15, abstract = {We present a parallel implementation of second-order Møller-Plesset perturbation theory with the resolution-of-the-identity approximation (RI-MP2). The implementation is based on a recent improved sequential implementation of RI-MP2 within the Turbomole program package and employs the message passing interface (MPI) standard for communication between distributed memory nodes. The parallel implementation extends the applicability of canonical MP2 to considerably larger systems. Examples are presented for full geometry optimizations with up to 60 atoms and 3300 basis functions and MP2 energy calculations with more than 200 atoms and 7000 basis functions.}, author = {H{\"{a}}ttig, Christof and Hellweg, Arnim and K{\"{o}}hn, Andreas}, doi = {10.1039/b515355g}, isbn = {1463-9076}, issn = {14639076}, journal = {Phys. Chem. Chem. Phys.}, number = 10, pages = {1159–1169}, pmid = {16633596}, title = {{Distributed memory parallel implementation of energies and gradients for second-order Møller-Plesset perturbation theory with the resolution-of-the-identity approximation}}, url = {http://dx.doi.org/10.1039/b515355g}, volume = 8, year = 2006 }
  Link
  http://dx.doi.org/10.1039/b515355g
3. Pecul, M., Pawłowski, F., Jørgensen, P., Köhn, A., & Hättig, C. “High-order correlation effects on dynamic hyperpolarizabilities and their geometric derivatives: A comparison with density functional results.” J. Chem. Phys. 124, 114101 (2006)
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  @article{AK16, abstract = {Second harmonic generation hyperpolarizabilities and their geometric derivatives have been calculated for HF, HCl, CO, and LiF, using the coupled cluster hierarchies, CCS, CC2, CCSD, CC3, and large correlation-consistent basis sets. The full configuration interaction results have been used to test the accuracy of the coupled cluster results. The CCS and CC2 methods do not improve on the Hartree-Fock results while CCSD is robust and gives significant improvements compared to CCS and CC2. The effects of triples in CC3 are in some cases substantial. Higher order correlation effects are significant for LiF. Including core-valence correlation effects is required only if high accuracy is desired. The coupled cluster results have been used as benchmarks for the results obtained by means of density functional theory using various exchange-correlation functionals. For the hyperpolarizability B3LYP was found to perform best, i.e., to give the results closest to the CC3 ones, while for the geometric derivatives none of the considered functionals was able to give a consistent description for all the considered molecules.}, author = {Pecul, Magdalena and Paw{\l}owski, Filip and Jørgensen, Poul and K{\"{o}}hn, Andreas and H{\"{a}}ttig, Christof}, doi = {10.1063/1.2173253}, isbn = {00219606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 11, pages = 114101, pmid = {16555868}, title = {{High-order correlation effects on dynamic hyperpolarizabilities and their geometric derivatives: A comparison with density functional results}}, url = {http://dx.doi.org/10.1063/1.2173253}, volume = 124, year = 2006 }
  Link
  http://dx.doi.org/10.1063/1.2173253
4. Hättig, C., Hellweg, A., & Köhn, A. “Intramolecular charge-transfer mechanism in quinolidines: The role of the amino twist angle.” J. Am. Chem. Soc. 128, 15672–15682 (2006)
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  @article{AK19, abstract = {Quantum-chemical calculations with the approximate coupled-cluster singles-and-doubles model CC2 have been carried out for 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6). For this molecule dual fluorescence was experimentally observed, raising the discussion about the importance of the amino twist angle for this process. The calculations suggest that both the ground state and the normal fluorescent state are significantly twisted by 30 degrees -40 degrees and that the molecule is flexible enough to move into an even stronger twisted conformation (60 degrees -70 degrees ) in its intramolecular charge-transfer (ICT) state which is responsible for the anomalous fluorescence band. Such a conformation both minimizes the total energy in the S1 state and maximizes the dipole moment. The barrier from the normal fluorescent state to the ICT state region is very small. Comparison to the situation in the 1-methyl-derivative NMC6 suggests that a large alkyl substituent makes the preferably planar normal fluorescent state energetically unfavorable compared to the ICT state and thus promotes the occurrence of dual fluorescence.}, author = {H{\"{a}}ttig, Christof and Hellweg, Arnim and K{\"{o}}hn, Andreas}, doi = {10.1021/ja0642010}, issn = {00027863}, journal = {J. Am. Chem. Soc.}, number = 49, pages = {15672–15682}, pmid = {17147377}, title = {{Intramolecular charge-transfer mechanism in quinolidines: The role of the amino twist angle}}, url = {http://dx.doi.org/10.1021/ja0642010}, volume = 128, year = 2006 }
  Link
  http://dx.doi.org/10.1021/ja0642010
5. Gaertner, B., Köhn, A., & Himmel, H.-J. “The First End-On Bonded Superoxo Complexes of Ga and In: The Oxygen-Rich Compounds GaO₄ and InO₄.” Eur. J. Inorg. Chem. 2006, 1496–1504 (2006)
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  @article{AK18, author = {Gaertner, Benjamin and K{\"{o}}hn, Andreas and Himmel, Hans-J{\"{o}}rg}, doi = {10.1002/ejic.200501116}, issn = {1434-1948}, journal = {Eur. J. Inorg. Chem.}, number = 7, pages = {1496–1504}, title = {{The First End-On Bonded Superoxo Complexes of Ga and In: The Oxygen-Rich Compounds GaO₄ and InO₄}}, url = {http://dx.doi.org/10.1002/ejic.200501116}, volume = 2006, year = 2006 }
  Link
  http://dx.doi.org/10.1002/ejic.200501116
2005
1. Høst, S., Jørgensen, P., Köhn, A., Pawłowski, F., Klopper, W., & Hättig, C. “Frequency-dependent hyperpolarizabilities of the Ne, Ar, and Kr atoms using the approximate coupled cluster triples model CC3.” J. Chem. Phys. 123, 94303 (2005)
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  @article{AK14, abstract = {The frequency-dependent electric field-induced second harmonic generation (ESHG) second hyperpolarizabilities gamma of neon, argon, and krypton are calculated using the approximate coupled cluster triples model CC3. Systematic basis set investigations are carried out to establish basis set limits, and scalar relativistic effects are accounted for by direct perturbation theory. To estimate higher-order correlation effects, full configuration-interaction results are used to benchmark the accuracy of CC3. The best theoretical estimates obtained thereby for the static second hyperpolarizabilities gamma(0) are 107.4, 1159, and 2589 a.u. for neon, argon, and krypton, respectively. These values as well as the results for the dispersion curve of the parallel component gamma( parallel) agree well with the latest experimental values from electric field-induced second harmonic generation. In addition, the dispersion of the perpendicular component gamma( perpendicular) and the hyperpolarizability ratios gamma( parallel)gamma( perpendicular) has been studied for the first time on a consistently correlated ab initio level. The analysis of the results indicates that, in particular for neon and krypton, the presently available experimental values are flawed.}, author = {Høst, Stinne and Jørgensen, Poul and K{\"{o}}hn, Andreas and Paw{\l}owski, Filip and Klopper, Wim and H{\"{a}}ttig, Christof}, doi = {10.1063/1.2008211}, isbn = {0021-9606 (Print)$\backslash$r0021-9606 (Linking)}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 9, pages = 94303, pmid = {16164341}, title = {{Frequency-dependent hyperpolarizabilities of the Ne, Ar, and Kr atoms using the approximate coupled cluster triples model CC3}}, url = {http://dx.doi.org/10.1063/1.2008211}, volume = 123, year = 2005 }
  Link
  http://dx.doi.org/10.1063/1.2008211
2. Köhn, A., Gaertner, B., & Himmel, H. J. “On the oxidation of gallium and indium: Characterization of the cyclic and linear GaO₂ and InO₂ molecules generated by the spontaneous and photoinduced reaction of Ga and in atoms with O2and determination of the reaction mechanism.” Chem. - A Eur. J. 11, 5575–5588 (2005)
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  @article{AK12, abstract = {In this work, the spontaneous and photolytically activated reactions of Ga and In atoms (M) with O2 (in Ar and solid O2) are studied with the aid of the matrix-isolation technique and the use of IR, Raman, and UV/Vis spectroscopy in combination with detailed quantum-chemical calculations. Vibrational spectra were recorded for several different isotopomers (69Ga, 71Ga, 16O2, 18O2, 16O18O). The results show that the spontaneously formed cyclic MO2 molecules photoisomerize to give the linear OMO molecules. The collected vibrational data were then used to characterize the bond properties of the linear OMO molecules in detail. The results are compared to those obtained for CO2(+) and neutral OEO compounds, where E is an element of Group 14. Quantum-chemical calculations were carried out at various levels of theory for GaO2. These calculations indicate that linear OMO is slightly more stable than its cyclic isomer. These calculations were also used to obtain information about the reaction mechanism, and show that the formation of the cyclic isomer from Ga atoms and O2 occurs without a significant barrier. Abrupt changes of the dipole moment and the O-O bond length during the approach of the O2 molecule toward the Ga atom mark the point on the potential energy surface at which one electron jumps from the Ga atom onto the O2 unit. The isomerization of cyclic GaO2 to the linear global minimum structure is accompanied by a significant barrier, which explains why this reaction requires photoactivation.}, author = {K{\"{o}}hn, Andreas and Gaertner, Benjamin and Himmel, Hans J{\"{o}}rg}, doi = {10.1002/chem.200500137}, isbn = {0947-6539 (Print)$\backslash$r0947-6539 (Linking)}, issn = {09476539}, journal = {Chem. - A Eur. J.}, number = 19, pages = {5575–5588}, pmid = {16013031}, title = {{On the oxidation of gallium and indium: Characterization of the cyclic and linear GaO₂ and InO₂ molecules generated by the spontaneous and photoinduced reaction of Ga and in atoms with O2and determination of the reaction mechanism}}, url = {http://dx.doi.org/10.1002/chem.200500137}, volume = 11, year = 2005 }
  Link
  http://dx.doi.org/10.1002/chem.200500137
3. Köhn, A., & Olsen, J. “Orbital-optimized coupled-cluster theory does not reproduce the full configuration-interaction limit.” J. Chem. Phys. 122, 84116 (2005)
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  @article{AK11, abstract = {It is shown that due to the mixing of the usual projection approach of coupled cluster with variational orbital optimization, orbital-optimized coupled cluster (OCC) fails to reproduce the full configuration-interaction (full CI) limit when the cluster operator becomes complete. It is pointed out that the fulfillment of the projected singles equations, which define the orbital gradient in Brueckner coupled cluster (BCC), is mandatory for a correct behavior. As numerical examples we present general OCC and BCC calculations up to the full CI limit on CH(2) and an active-space model of ozone. The observed deviations of OCC from full CI are of the order of the correlation error obtained in calculations with up to quadruples excitations. Thus the failure of OCC may be considered tolerable in more approximate calculations but clearly prohibitive for any benchmark application. For applications to active-space models a hybrid approach for OCC is suggested in which for active particle-hole rotations the Brueckner orbital gradient is employed, whereas for the remaining orbital rotations the variational orbital gradient is retained.}, author = {K{\"{o}}hn, Andreas and Olsen, Jeppe}, doi = {10.1063/1.1850918}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 8, pages = 84116, pmid = {15836029}, title = {{Orbital-optimized coupled-cluster theory does not reproduce the full configuration-interaction limit}}, url = {http://dx.doi.org/10.1063/1.1850918}, volume = 122, year = 2005 }
  Link
  http://dx.doi.org/10.1063/1.1850918
4. Thøgersen, L., Olsen, J., Köhn, A., Jørgensen, P., Sałek, P., & Helgaker, T. “The trust-region self-consistent field method in Kohn-Sham density-functional theory.” J. Chem. Phys. 123, 74103 (2005)
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  @article{AK13, abstract = {The trust-region self-consistent field (TRSCF) method is extended to the optimization of the Kohn-Sham energy. In the TRSCF method, both the Roothaan-Hall step and the density-subspace minimization step are replaced by trust-region optimizations of local approximations to the Kohn-Sham energy, leading to a controlled, monotonic convergence towards the optimized energy. Previously the TRSCF method has been developed for optimization of the Hartree-Fock energy, which is a simple quadratic function in the density matrix. However, since the Kohn-Sham energy is a nonquadratic function of the density matrix, the local energy functions must be generalized for use with the Kohn-Sham model. Such a generalization, which contains the Hartree-Fock model as a special case, is presented here. For comparison, a rederivation of the popular direct inversion in the iterative subspace (DIIS) algorithm is performed, demonstrating that the DIIS method may be viewed as a quasi-Newton method, explaining its fast local convergence. In the global region the convergence behavior of DIIS is less predictable. The related energy DIIS technique is also discussed and shown to be inappropriate for the optimization of the Kohn-Sham energy.}, author = {Thøgersen, Lea and Olsen, Jeppe and K{\"{o}}hn, Andreas and Jørgensen, Poul and Sa{\l}ek, Pawe{\l} and Helgaker, Trygve}, doi = {10.1063/1.1989311}, isbn = {0021-9606 (Print)$\backslash$r0021-9606 (Linking)}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 7, pages = 74103, pmid = {16229555}, title = {{The trust-region self-consistent field method in Kohn-Sham density-functional theory}}, url = {http://dx.doi.org/10.1063/1.1989311}, volume = 123, year = 2005 }
  Link
  http://dx.doi.org/10.1063/1.1989311
2004
1. Köhn, A., & Hättig, C. “On the nature of the low-lying singlet states of 4-(Dimethyl-amino)benzonitrile.” J. Am. Chem. Soc. 126, 7399–7410 (2004)
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  @article{AK10, abstract = {4-(N,N-Dimethyl-amino)benzonitrile (DMABN) is a prototype molecule for dual fluorescence. The anomalous emission has been attributed to an intramolecular charge-transfer (ICT) state, and the structure of the latter is still subject to some controversy. We applied a recently developed analytical gradient code for the approximate coupled-cluster singles-and-doubles method CC2 in combination with accurate basis sets to address this problem. Fully optimized excited state structures are presented for the ICT state and the so-called locally excited state, and recent transient IR and Raman measurements on the excited states are interpreted by means of calculated harmonic frequencies. Strong evidence is found for an electronic decoupling of the phenyl and the dimethyl-amino moiety, resulting in a minimum structure for the ICT state with a twisted geometry. In contrast to previous findings, the structure of this state is, at least in the gas phase, not C(2v) symmetric but distorted towards C(s) symmetry. The distortion coordinate is a pyramidalization of the phenyl carbon atom carrying the dimethyl-amino group. The results from the CC2 model are supported by single-point calculations using more elaborate coupled-cluster models (CCSD, CCSDR(3)) and by CASSCF calculations.}, author = {K{\"{o}}hn, Andreas and H{\"{a}}ttig, Christof}, doi = {10.1021/ja0490572}, issn = {00027863}, journal = {J. Am. Chem. Soc.}, number = 23, pages = {7399–7410}, pmid = {15186180}, title = {{On the nature of the low-lying singlet states of 4-(Dimethyl-amino)benzonitrile}}, url = {http://dx.doi.org/10.1021/ja0490572}, volume = 126, year = 2004 }
  Link
  http://dx.doi.org/10.1021/ja0490572
2003
1. Fliegl, H., Köhn, A., Hättig, C., & Ahlrichs, R. “Ab initio calculation of the vibrational and electronic spectra of trans- and cis-azobenzene.” J. Am. Chem. Soc. 125, 9821–9827 (2003)
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  @article{AK08, abstract = {We report accurate geometries and harmonic force fields for trans- and cis-azobenzene determined by second-order Møller-Plesset perturbation theory. For the trans isomer, the planar structure with C(2h) symmetry, found in a recent gas electron diffraction experiment, is verified. The calculated vibrational spectra are compared with experimental data and density functional calculations. Important vibrational frequencies are localized and discussed. For both isomers, we report UV spectra calculated using the second-order approximate coupled-cluster singles-and-doubles model CC2 with accurate basis sets. Vertical excitation energies and oscillator strengths have been determined for the lowest singlet n(pi)* and (pi)(pi)* transitions. The results are compared with the available experimental data and second-order polarization propagator (SOPPA) and density functional (DFT) calculations. For both isomers, the CC2 results for the excitation energies into the S(1) and S(2) states agree within 0.1 eV with experimental gas-phase measurements.}, author = {Fliegl, Heike and K{\"{o}}hn, Andreas and H{\"{a}}ttig, Christof and Ahlrichs, Reinhart}, doi = {10.1021/ja034433o}, isbn = {0002-7863}, issn = {00027863}, journal = {J. Am. Chem. Soc.}, number = 32, pages = {9821–9827}, pmid = {12904048}, title = {{Ab initio calculation of the vibrational and electronic spectra of trans- and cis-azobenzene}}, url = {http://dx.doi.org/10.1021/ja034433o}, volume = 125, year = 2003 }
  Link
  http://dx.doi.org/10.1021/ja034433o
2. Köhn, A., & Hättig, C. “Analytic gradients for excited states in the coupled-cluster model CC2 employing the resolution-of-the-identity approximation.” J. Chem. Phys. 119, 5021–5036 (2003)
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  @article{AK09, abstract = {The derivation and implementation of excited state gradients is reported for the approximate coupled-cluster singles and doubles model CC2 employing the resolution-of-the-identity approximation for electron repulsion integrals. The implementation is profiled for a set of examples with up to 1348 basis functions and exhibits no I/O bottlenecks. A test set of sample molecules is used to assess the performance of the CC2 model for adiabatic excitation energies, excited state structure constants and vibrational frequencies. We find very promising results, especially for adiabatic excitation energies, though the need of a single-reference ground state and a single-replacement dominated excited state puts some limits on the applicability of the method. Its reliability, however, can always be tested on grounds of diagnostic measures. As an example application, we present calculations on the π∗←π excited state of trans-azobenzene.{\textcopyright} 2003 American Institute of Physics.}, author = {K{\"{o}}hn, Andreas and H{\"{a}}ttig, Christof}, doi = {10.1063/1.1597635}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 10, pages = {5021–5036}, pmid = {19527681}, title = {{Analytic gradients for excited states in the coupled-cluster model CC2 employing the resolution-of-the-identity approximation}}, url = {http://dx.doi.org/10.1063/1.1597635}, volume = 119, year = 2003 }
  Link
  http://dx.doi.org/10.1063/1.1597635
3. Köhn, A., Himmel, H. J., & Gaertner, B. “Why does a Ga₂ dimer react spontaneously with H₂, but a Ga atom does not? - A detailed quantum chemical investigation of the differences in reactivity between Ga atoms and Ga₂ dimers, in combination with experimental results.” Chem. - A Eur. J. 9, 3909–3919 (2003)
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  @article{AK07, abstract = {The spontaneous and photoactivated reactions between Ga(2) and H(2) in a matrix of solid Ar at 12 K have been followed by using IR spectroscopy and have been shown to give access to several isomers of the subvalent hydride Ga(2)H(2). We now present Raman spectra for this system, to complete its characterization on the basis of vibrational spectra. In addition, the differences between the reactivity of a Ga atom and a Ga(2) dimer toward H(2) are evaluated. The matrix isolation experiments have shown that Ga(2) reacts spontaneously with H(2,) at 12 K, to give the cyclic subvalent hydride Ga(micro-H)(2)Ga (D(2h) symmetry), which can be transformed into two other isomers of Ga(2)H(2) by selective photoactivation. Interestingly, the spontaneous reaction is subject to a marked isotopic effect. In total, the experimental results provide detailed information about the reaction mechanism. In contrast to Ga(2), Ga atoms do not react spontaneously with H(2); on photoactivation they instead yield the radical species GaH(2). The quantum chemical calculations presented herein start with an analysis of the structures and relative energies of the relevant species at the MP2 level, by using extended basis sets, and lead on to a discussion of the correlation diagrams for both reactions. Finally, CASSCF and MRCI methods, in combination with moderate-sized basis sets, were employed to analyze in detail the mechanisms of the two reactions. It will be shown that the computational results, in concert with the experimental findings, provide a satisfying explanation of the contrasting reactivities of Ga and Ga(2).}, author = {K{\"{o}}hn, Andreas and Himmel, Hans J{\"{o}}rg and Gaertner, Benjamin}, doi = {10.1002/chem.200204633}, isbn = {0947-6539 (Print)$\backslash$r0947-6539 (Linking)}, issn = {09476539}, journal = {Chem. - A Eur. J.}, number = 16, pages = {3909–3919}, pmid = {12916117}, title = {{Why does a Ga₂ dimer react spontaneously with H₂, but a Ga atom does not? - A detailed quantum chemical investigation of the differences in reactivity between Ga atoms and Ga₂ dimers, in combination with experimental results}}, url = {http://dx.doi.org/10.1002/chem.200204633}, volume = 9, year = 2003 }
  Link
  http://dx.doi.org/10.1002/chem.200204633
2002
1. Köhn, A., & Hättig, C. “Comment on ’efficient calculation of canonical MP2 energies ’［P. Pulay, S. Saebø, K. Wolinski, Chem. Phys. Lett. 344 (2001) 543-552］.” Chem. Phys. Lett. 358, 350–353 (2002)
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  @article{AK05, abstract = {Accuracy and efficiency of the resolution of the identity approximation in second-order Møller-Plesset perturbation theory (RI-MP2) are reassessed in the light of the new MP2 code by Pulay et al. It is demonstrated that with the RI approximation considerable computational savings can be achieved, especially for larger basis sets of triple-zeta and higher quality. The error introduced by the RI approximation has to be viewed in comparison with the one-electron basis set error. It is shown that this error is insignificant if optimized auxiliary basis sets are used. {\textcopyright} 2002 Published by Elsevier Science B.V.}, author = {K{\"{o}}hn, Andreas and H{\"{a}}ttig, Christof}, doi = {10.1016/S0009-2614(02)00609-7}, issn = {00092614}, journal = {Chem. Phys. Lett.}, number = {3-4}, pages = {350–353}, title = {{Comment on 'efficient calculation of canonical MP2 energies '［P. Pulay, S. Saebø, K. Wolinski, Chem. Phys. Lett. 344 (2001) 543-552］}}, url = {http://dx.doi.org/10.1016/S0009-2614(02)00609-7}, volume = 358, year = 2002 }
  Link
  http://dx.doi.org/10.1016/S0009-2614(02)00609-7
2. Weigend, F., Köhn, A., & Hättig, C. “Efficient use of the correlation consistent basis sets in resolution of the identity MP2 calculations.” J. Chem. Phys. 116, 3175–3183 (2002)
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  @article{AK03, abstract = {The convergence of the second-order Møller–Plesset perturbation theory (MP2) correlation energy with the cardinal numberX is investigated for the correlation consistent basis-set series cc-pVXZ and cc-pV (X+d) Z . For the aug-cc-pVXZ and aug-cc-pV (X+d) Z series the convergence of the MP2 correlation contribution to the dipole moment is studied. It is found that, when d-shell electrons cannot be frozen, the cc-pVXZ and aug-cc-pV X Z basis sets converge much slower for third-row elements then they do for first- and second-row elements. Based on the results of these studies criteria are deduced for the accuracy of auxiliary basis sets used in the resolution of the identity (RI) approximation for electron repulsion integrals. Optimized auxiliary basis sets for RI-MP2 calculations fulfilling these criteria are reported for the sets cc-pVXZ, cc-pV (X+d) Z , aug-cc-pVXZ, and aug-cc-pV (X+d) Z with X= D , T, and Q. For all basis sets the RI error in the MP2 correlation energy is more than two orders of magnitude smaller than the usual basis-set error. For the auxiliary aug-cc-pVXZ and aug-cc-pV (X+d) Z sets the RI error in the MP2 correlation contribution to the dipole moment is one order of magnitude smaller than the usual basis set error. Therefore extrapolations towards the basis-set limit are possible within the RI approximation for both energies and properties. The reduction in CPU time obtained with the RI approximation increases rapidly with basis set size. For the cc-pVQZ basis an acceleration by a factor of up to 170 is observed.}, archiveprefix = {arXiv}, arxivid = {arXiv:1107.3385v1}, author = {Weigend, Florian and K{\"{o}}hn, Andreas and H{\"{a}}ttig, Christof}, doi = {10.1063/1.1445115}, eprint = {arXiv:1107.3385v1}, isbn = {doi:10.1063/1.1445115}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 8, pages = {3175–3183}, title = {{Efficient use of the correlation consistent basis sets in resolution of the identity MP2 calculations}}, url = {http://dx.doi.org/10.1063/1.1445115}, volume = 116, year = 2002 }
  Link
  http://dx.doi.org/10.1063/1.1445115
3. Hättig, C., Köhn, A., & Hald, K. “First-order properties for triplet excited states in the approximated coupled cluster model CC2 using an explicitly spin coupled basis.” J. Chem. Phys. 116, 5401–5410 (2002)
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  @article{AK04, abstract = {An implementation is reported for first-order properties of excited triplet states within the approximate coupled cluster model CC2 using an explicitly spin coupled basis for the triplet excitation manifold and the resolution of the identity (RI) approximation for the electron repulsion integrals. Results are presented for the change of the second moment of charge upon excitation in the ππ∗ valence and n = 3 Rydberg states of benzene. Employing large basis sets with up to 828 functions, we obtain results close to the CC2 basis set limit and are able to resolve an uncertainty in the assignment of the lowest 1E1u states. It is found that the often used {\%}T1 measure for the single excitation contribution to excited states is not reliable for a comparison across different excitation operator manifolds. An alternative diagnostic is proposed which provides a unique measure for the single excitation contribution that is independent of the chosen representation of the excitation operator manifold. {\textcopyright} 2002 American Institute of Physics.}, author = {H{\"{a}}ttig, Christof and K{\"{o}}hn, Andreas and Hald, Kasper}, doi = {10.1063/1.1457434}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 13, pages = {5401–5410}, title = {{First-order properties for triplet excited states in the approximated coupled cluster model CC2 using an explicitly spin coupled basis}}, url = {http://dx.doi.org/10.1063/1.1457434}, volume = 116, year = 2002 }
  Link
  http://dx.doi.org/10.1063/1.1457434
4. Hättig, C., & Köhn, A. “Transition moments and excited-state first-order properties in the coupled-cluster model CC2 using the resolution-of-the-identity approximation.” J. Chem. Phys. 117, 6939–6951 (2002)
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  @article{AK06, abstract = {An implementation of transition moments and excited-state first-order properties is reported for the approximate coupled-cluster singles-and-doubles model (CC2) using the resolution of the identity (RI) approximation. In parallel to the previously reported code for the ground- and excited-state amplitude equations, we utilize a partitioned form of the CC2 equations and thus eliminate the need to store any N4 intermediates. This opens the perspective for applications on molecules with 30 and more atoms. The accuracy of the RI approximation is tested for a set of 29 molecules for the aug-cc-pVXZ (X = D,T,Q) basis sets in connection with the recently optimized auxiliary basis sets. These auxiliary basis sets are found to be sufficient even for the description of diffuse states. The RI error is compared to the usual basis set error and is demonstrated to be insignificant. {\textcopyright} 2002 American Institute of Physics.}, author = {H{\"{a}}ttig, Christof and K{\"{o}}hn, Andreas}, doi = {10.1063/1.1506918}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 15, pages = {6939–6951}, title = {{Transition moments and excited-state first-order properties in the coupled-cluster model CC2 using the resolution-of-the-identity approximation}}, url = {http://dx.doi.org/10.1063/1.1506918}, volume = 117, year = 2002 }
  Link
  http://dx.doi.org/10.1063/1.1506918
2001
1. Köhn, A., Weigend, F., & Ahlrichs, R. “Theoretical study on clusters of magnesium.” Phys. Chem. Chem. Phys. 3, 711–719 (2001)
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  @article{AK02, abstract = {The energetic and structural properties of neutral magnesium clusters Mg(n) (n = 2 to 22 and selected clusters up to 309) are investigated by means of density functional theory. The small clusters of sizes n = 4, 10 and 20 show enhanced stability, which is in line with the predictions of the shell model. Larger clusters were selected according to the principle of geometric shell closings; different kinds of shapes and packing types were considered. Among the cluster sizes studied, icosahedral packings are most favourable, which supports the experimental findings. Weak evidence is found for hcp packings to become most stable for large n. The calculated cohesive energies lead to an extrapolated bulk value of 1.4 eV, in reasonable agreement with experiment (1.51 eV). The electronic shell structure of large clusters is discussed in terms of the shell model, which is found to be reliable for up to 40 electrons, while the stability of higher shells is very dependent on the cluster shape.}, author = {K{\"{o}}hn, A. and Weigend, F. and Ahlrichs, R.}, doi = {10.1039/b007869g}, isbn = {1463-9076}, issn = {14639076}, journal = {Phys. Chem. Chem. Phys.}, number = 5, pages = {711–719}, title = {{Theoretical study on clusters of magnesium}}, url = {http://dx.doi.org/10.1039/b007869g}, volume = 3, year = 2001 }
  Link
  http://dx.doi.org/10.1039/b007869g
2000
1. Schooss, D., Gilb, S., Kaller, J., Kappes, M. M., Furche, F., Köhn, A., May, K., & Ahlrichs, R. “Photodissociation spectroscopy of Ag₄⁺(N₂)ₘ, m = 0-4.” J. Chem. Phys. 113, 5361–5371 (2000)
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  @article{AK01, abstract = {We have determined photodissociation spectra of Ag-4(+)(N- 2)(m=0-4) in the photon energy range from 2.1 to 3.75 eV. The cluster complexes were generated by laser vaporization and their absolute photodissociation cross sections were measured in depletion using a modified reflectron time-of-flight mass spectrometer. The band spectra show significant nonmonotonic variations with increasing N-2 coverage. The experimental data are contrasted with quantum chemical calculations of ground and electronically excited states using density functional as well as post-Hartree-Fock methods. Experiment and theory are in qualitative agreement allowing tentative assignment of the depletion spectra and insight into the relevant bonding interactions. (C) 2000 American Institute of Physics. ［S0021- 9606(00)01137-5］.}, author = {Schooss, Detlef and Gilb, Stefan and Kaller, Julian and Kappes, Manfred M. and Furche, Filipp and K{\"{o}}hn, Andreas and May, Klaus and Ahlrichs, Reinhart}, doi = {10.1063/1.1290028}, isbn = {0021-9606}, issn = {00219606}, journal = {J. Chem. Phys.}, number = 13, pages = {5361–5371}, title = {{Photodissociation spectroscopy of Ag₄⁺(N₂)ₘ, m = 0-4}}, url = {http://dx.doi.org/10.1063/1.1290028}, volume = 113, year = 2000 }
  Link
  http://dx.doi.org/10.1063/1.1290028

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2024

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2022

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