from left to right: Marius Frank, Christof Hättig, Özlem Yönder, Niklas Süzner, Sarah Karbalei Khani, Alireza Marefat Khah, Chetana Badala Viswanatha, Hang Choi, Lisa Götte, Apostolos Minopetras

Our group is working on a number of projects concerned with the accurate description of interactions between molecules and of molecules with surfaces, solvents and electric or magnetic fields (i.e. spectroscopy), as well as accurate reaction and activation energies for applications in heterogeneous catalysis and reaction kinetics:

Electronic Excitations: Molecular Spectra and Structures of Excited States

Solvation, Weak Molecular Interactions and There Influence on Properties, Spectra and Reactivity

Heterogenous Catalysis: Interactions of Molecules with Surfaces

Interactions with Electromagnetic Fields: Linear and Nonlinear Optical Properties

Coupled-Cluster Response Theory: Ab Initio Methods for Frequency-dependent Molecular Properties

Our main tools for these investigations are is the well-known quantum chemistry package TURBOMOLE, to which we also contribute as a development group, and the quantum chemistry packages DALTON, CFOUR, and Molpro.

For further information you can also browse the list of our publications.

the transition energies to the lowest excited states in trans- and cis-azobenzene and there change on substitution in different positions with a variety of functional groups

the equilibrium structures of the lowest excited states of 4-(dimethyl-amino)-benzonitrile (DMABN) and the related NMC6 and NTC6, which are prototype molecules for the investigation and understanding of the so-called dual fluorescence phenomenon

UV/VIS spectra of chlorophyll chromophores and there change with chemical modification and/or complexation and the influence of the chlorophyll—chlorophyll interaction as it is in natural photosystems.

In cooperation with the group of Prof. Dr. Samual Leutwyler in Bern we benchmarked the accuracy of quantum chemical methods for the prediction of 0–0 transitions of aromatic organic molecules

The CC2 model is an approximated coupled-cluster singles-and-doubles (CCSD) method which has been proposed in 1996 by Christiansen, Jørgensen and Koch for response calculations on molecules which are out of reach for CCSD and higher correlated methods. It is one of the simplest correlated ab initio methods for excited states and yields energies for singly-excited states which are correct through second-order in the electron-electron interaction (dynamic electron correlation), as the well-known second-order Møller-Plesset perturbation theory (MP2) does for ground states. It is thus well-suited for the study of excited states of large closed-shell (or at least "single-reference") molecules. In difference to the perturbative doubles corrections CIS(D) to the widely used configuration interaction singles (CIS) method and similar perturbative approaches to excited states which use non-degenerate perturbation theory, CC2 is not limited to energetically isolated states. A feature, which is important in the search of excited state equilibrium structures. In several applications CC2 has been shown to be a viable tool for such studies.

As MP2 and other related methods based on a second-order treatment of electron correlation, CC2 can be implemented very efficiently with a so-called resolution of the identity approximation for the integrals which describe the electron-electron interaction and thereby made applicable to relatively large molecules, which have been intractable with conventional implementations. As demonstrated in the mid 1990's by Weigend and Häser for MP2, the computationally costs and demands (CPU time, memory and disk space) are for most applications reduced by orders of magnitudes.

During the last years we have in our group developed the `ricc2`
code of the Turbomole package, an implementation of CC2 with the
resolution-of-the-identity approximation which includes

a distributed memory parallel implementation based on the MPI standard

an implementation of the SOS variants of MP2, CC2 and ADC(2) with O(N

^{4}) scaling computational costs for ground and excitation energies and gradients, transitions moments and first-order propertiesstatic and frequency-dependent polarizabilities for ground states and electronically excited states

an embedding in an polarizable environment for the prediction of solvent and other environment effects of excited states and spectra including two-photon spectra

analytic molecular Hessians and dipole gradients for vibrational frequencies and IR intensities

oscillator strengths for triplet excited states induced by spin-orbit coupling to compute phosphorescence lifetimes

As a side product the code includes a revised implementation of RI-MP2 for ground state energies and gradients and implementations of RI-CIS(D) and RI-ADC(2) (algebraic diagrammatic construction through second order, J. Schirmer 1981) for excitation energies. All functionalities at the MP2, CC2, CIS(D) and ADC(2) levale are implemented for closed-shell and unrestricted Hartree-Fock references and most of them are parallelized for PC clusters using the Message Passing Interface (MPI) standard.

Solvation effects on UV/Vis absorption spectra and two-photon spectra

Prediction of the optical rotation of molecules in gas phase and solution by ab initio calculations

Theoretical prediction of nonlinear optical properties of molecular crystals

accurate ab initio calculations of van der Waals dispersion coefficients

The description of intermolecular interactions between polyatomic molecules through distributed multipole moments, polarizabilities and van der Waals dispersion coefficients

Pressure-dependence of linear and nonlinear optical properties of rare gases

The synthesis of methanol from CO and H2 by heterogenous catalysis, is one of the most important processes in chemical industry. A large quantity of methanol is produced annually using the multicomponent Cu/ZnO/Al2O3 catalyst and CO2/CO/H2 as the feed gas. Within the collaborative research center (SFB 558) with studied the reaction pathways for the methanol synthesis at oxygen vacancies at the polar 000-1 ZnO surface. Starting from optimized structures for educts and products, we performed Growing-String Newton-Trajectory calculations, in the framework of the embbed cluster approach, to obtain initial guesses for possible reaction pathways. To refine this reaction pathways, we did subsequent Nedged Elastic Band calculations. The structures with the highest energy along the "elastic band" give a first approximation for the transition state structures, which we are refine using a Trust-Region-Image-Minimization algorithm. The optimized structures for the transition state as well as the sturctures for products and educts are check by evaluating the forces constant (Hessian) matrices. From latter we also calculate the zero point vibrational energies within the harmonic approximation. In this way we also obtain IR-Spectra for the possible intermediates and free enthaplies. By comparison of such "simulated" vibrational spectra and enthalpies with experimental IR and HR-EELS data (obtained within the collaborative research center) we are able to identify and characterize intermediates and adsorbates on the surface and gain insight into reaction and adsorbtion processes.

Methanol Synthesis at an Oxygen Vacancy on the Polar ZnO(000-1) Surface

Formation of weakly bound, ordered adlayers of CO on rutile TiO2(110)

The interaction of molecules with electromagnetic fields
(homogenous or inhomogeneous, static or time-dependent) are related
to a large variety of important molecular properties. The most
well-known ones are the permanent dipole moment and the dipole
polarizability, which describe the change of the energy in an
homogenous electric field through first- and second-order in the
field strength: *E = f · μ + f ^{2} · α *.
In higher orders the interaction with electric fields is
described by hyperpolarizabilities and
— if magnetic fields are involved —
magnetizabilities and hypermagnetizabilities, which are
responsible for various nonlinear (magneto-) optical effects

Kerr effect

Pockels effect

second and higher harmonic generation

intensity-dependent refractive index

Verdet effect

Faraday effect

Buckingham effect

Cotton-Mouton effect

With the availability of strong lasers and magnets, the accurate knowledge of these nonlinear (magneto-) optical properties became essential for the understanding and the prediction of the behavior of molecules in strong fields. For some of these effects, however, accurate quantative measurements are difficult and/or only possible relative to a reference substance. Quantum chemical calculations are here of great help for a better understanding of these properties and for the validation of experimental results. Highly accurate ab initio calculations can serve for calibration of measurements. This has been the motivation for a number of investigations we have carried out in collaboration with international partners, in particular the Theoretical Chemistry Group at Århus University, Denmark:

Many of these properties are related to intermolecular interactions (which
are also of electric or electromagnetic nature) and there influence
on molecular properties. E.g. the well-known van der Waals *C _{6}*
dispersion coefficients can be obtained from the frequency-dependent
dipole polarizability.

Driven by the interest in frequency-dependent molecular properties, theoretical spectroscopy and intermolecular interactions, the development of ab initio, in particular coupled-cluster, methods for the theoretical description of the interaction of molecules with oscillating (i.e. time-dependent) fields has become a central topic of our research. The basis of this work is a modern formulation of response theory based on a time-dependent Lagrangian, which provides a general handle for the description of a physical system interacting with time-dependent external fields in approximate wavefunction models. Most our work in this field has been carried out in collaboration with group of Poul Jørgensen at Århus University and other developers of the Dalton program and is included in the coupled-cluster response code which is part of Dalton. Our work in this field has been concerned with

the implementation of the approximate coupled-cluster singles, doubles and triples model CC3 for frequency-dependent first and second hyperpolarizabilities, which allows now highly accurate calculations of these nonlinear optical properties

the development of a coupled-cluster response program which employs an explicitly correlated R12 or F12 ansatz for the wavefunction to increase the accuracy and/or reduce the computational costs for such calculations. This work was carried out in collaboration with Wim Klopper and coworkers (Institute of Physical Chemistry, University of Karlsruhe and Institute of Nanotechnology, Forschungszentrum Kalrsruhe) as a project of the DFG priority program 1145 "Modern and universal

*first-principles*methods for many-electron systems in chemistry and physics".