Fields of Research

Research areas of the Rauhut Group

Our research focusses mainly on the development of accurate quantum chemical methods as needed for a reliable prediction of spectroscopic properties of small molecules and molecular clusters (<25 atoms). It is the efficient and automated simulation of infrared, Raman, resonance Raman, photoelectron or photoionization spectra within a fully anharmonic framework and the account of high-order correlation effects, which dominates our work. All methods developed in our group are available in the Molpro program package.

Method development

Automated construction of multi-dimensional potential energy surfaces

The accurate and efficient determination of multi-dimensional potential energy surfaces with one or several local minima based on high-level single and multi-reference ab initiomethods is a non-trivial task, but is needed for the evaluation of vibrational wavefunctions or quantum-dynamical calculations. We develop techniques to reduce the computational effort while retaining very high accuracy [1]. A collection of highly accurate potential energy surfaces is provided in our PES database

Automated construction of multi-dimensional potential energy surfaces

Calculation of vibrational wavefunctions

We develop and use single and multi-reference self-consistent field methods (VSCF and VMCSCF [2,3]) to determine vibrational wavefunctions and vibrationally averaged molecular properties. This work is based on the full Watson-Hamiltonian [4].

Vibration correlation effects

We use methods in analogy to their well-known counterparts in electronic structure theory in order to account for correlation effects. These methods comprise single and multi-reference variational and perturbational approaches, as for example VCI [5], VMRCI [6], VMP2, VMRPT2 [6].

2nd order vibrational perturbation theory

Besides variational methods, we have also an implementation of VPT2 theory, which allows us to to determine vibrational transition energies and many vibrational constants at this level.

Spectra relying on more than one potential energy surface (c)
Spectra relying on more than one potential energy surface

Spectra relying on more than one potential energy surface

Photoelectron or photoionization spectra require the knowledge of more than one PES. Methods are developed, which allow for the efficient calculation of Franck-Condon factors or optical band shapes including Duschinsky effects [7]. All methods rely on highly correlated and fully anharmonic vibrational wavefunctions.

Vibrational spectra (c)
Vibrational spectra

Applications

Calculation of accurate infrared and Raman spectra

In collaboration with experimentally working groups we compute and analyze complex vibrational spectra of molecules and molecular clusters. In the last years our focus has been on rather instable species, which often show strong Fermi resonances [8,9]. For small systems the remaining error bar in our calculations is typically in the range of 2-3 wavenumbers, which of course increases for larger systems or high lying vibrational states.

Tunneling splittings (c)
Tunneling splittings

Calculation of tunneling splittings in double-well potentials

The accurate determination of tunneling splittings of excited vibrational states requires highly accurate potential energy surfaces and converged vibration correlation calculations [10]. Currently our approach is limited to rather small molecules and we are working on strategies to extend this to larger systems.

Calculation of photoelectron spectra (c)
Calculation of photoelectron spectra

Calculation of photoelectron and photoionization spectra

The prediction of photoelectron spectra is tedious due the calculation of excited state Born-Oppenheimer surfaces. Besides that the Duschinsky rotation connecting the two surfaces leads to further complications. We compute such spectra beyond the harmonic approximation based on highly correlated wavefunction. Besides that we investigate alternative routes to determine optical bandshapes making use of time-independent Raman wavefunctions.

Calculation of molecular properties

In our collaborations we a frequently asked if we can provide vibrationally averaged molecular properties as for example geometrical parameters, rotational constants, spectroscopic constants in general or vibrationally averaged dipole moments. Yes, we can – at very high levels.