The Quantum Chemistry Group (Hättig Research Group)

Hattig Research Group

At our 2018 office outing to Kletterwald Wetter. From left to right: Lisa Warczinski, Alireza Marefat Khah, Sarah Karbalei Khani, Christof Hättig, Marius Frank, Özlem Yönder, Julian Golzwarden

Here you find old group photos

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:

We are involved in the RESOLV Cluster of Excellence, the Center of Solvation Science ZEMOS. and the Research Department Solvation Science at the Ruhr University Bochum. We contribute with research projects to the Collaborative Research Centers CRC/TRR 129 Oxyflame "Development of methods and models to describe solid fuel reactions within an oxy-fuel atmosphere", CRC/TRR 247 "Heterogeneous Oxidation Catalysis in the Liquid Phase – Mechanisms and Materials in Thermal, Electro-, and Photocatalysis", and the DFG Priority Program SPP 1807 "Control of London dispersion interactions in molecular chemistry".

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.


Electronic Excitations: Molecular Spectra and Structures of Excited States

dmabn-curve Electronically excited states have usually a complicated electronic structure, and often also the molecular geometry is different from that in the ground state and difficult to predict. Chemical calculations are therefore an important tool for the understanding of molecular spectra and the photochemical reactivity of molecules. Modern electronic structure methods, as e.g. density functional and coupled-cluster response methods allow today to investigate with ab initio methods excited states of relatively large molecules, to predict accurately their spectra, and to determine the equilibrium structures of excited states. In our group we apply for these investigations (in addition to TDDFT) mainly the RI-CC2 approach, which has been developed in the group and is well-suited for applications on medium sized and large organic chromophores (up to ca. 100 atoms). Examples which have been studied with this approach in our group are:


The ricc2 Code: Correlated Ab Initio Methods for the Description of Electronic Excitations in Large Molecules

TURBOMOLE

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

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

SOLVATION@RUB Apart from laboratory measurements carried out in ultra high vacuum (and the interstellar space) molecules don't appear as isolated species but in a chemical environment where the interaction with other molecules influences their properties, structure and reactivity. The interaction between molecules is of electric nature, governed by the simple Coulomb law V(r) = Z1 Z2/(4πε0 r) for the interaction between charged particles. But because of the many possible ways how the electric interaction can become apparent, e.g. as interaction between static electric (e.g. dipole) moments, or between static moments and (through the polarizabilities and hyperpolarizabilities) induced moments, or as van der Waals dispersion interaction the detailed description of intermolecular interactions in terms of molecular properties is usually rather complex. Together with the weakness of these interactions, this makes the determination, understanding and prediction of the potential energy surfaces and the structures and spectra of van der Waals complexes and the influence of intermolecular interactions on the chemical and physical properties and chemical reactions a rich and challenging field. Because of their electric nature intermolecular interactions are intimately connected with the electric or optical molecular properties and response theory. Some problems which have been studied by us in collaboration with various partners are:


Oxy-Fuel Combustion: Char Combustion Kinetics

reaction pathway, click to enlarge

Since the Industrial Revolution, most of our energy consumption has been originating from fossil fuels. However, the conventional air-blown combustion of fossil fuels results in the release of significant amounts of carbon into the atmosphere. This problem lead to the development of Carbon Capture and Storage (CCS) technologies as a solution. Oxy-fuel combustion, using oxygen mixed with recirculated flue gas from the furnace, is a promising CCS technology to separate CO2. In order to better understand the combustion process under oxy-fuel conditions, we investigate char combustion kinetics in project-A7 "atomistic multiscale simulation of char combustion" of the collaborative research center CRC/TRR 129 Oxyflame.

We aim to build a reaction network model for char burnout. We use density functional theory (DFT) and find the approximate reaction pathways for the relevant reactions using the woelfling code. We are also planning to calculate adsorption energies of the gases in the combustion medium such as O2, CO, CO2, and H2O. The final kinetic model will be based on the energies of the DFT optimized structures calculated using the CCSD(F12*)(T) method and pair natural orbitals.


Heterogenous Catalysis: Interactions of Molecules with Surfaces

H<sub>2</sub> on a Pd cluster with carbon support, click to enlarge

Heterogeneous catalysis is one of the key technologies in the chemical industry. Many economically important chemical processes are heterogeneous catalyzed. The most common heterogenoeus catalysts are metal-nanoparticles with porous oxide or carbon as support. Up to the present, the elementary steps and the parameters which determine the reactivity are unknown for many of such heterogeneous catalyzed reactions. For supported metal-nanoparticles the interactions between the support material and the nanoparticles has only for a few examples been studied satisfyingly. Furthermore, a lot of phenomena observed in experimental studies like the influence of surface defects and the hydrogen spillover mechanism cannot be explained so far.

Our group has a longtime experience with computational studies on the electronic structure of surface defects in the context of heterogeneous catalysis. Within the collaborative research center (SFB 558) we studied between 2006 and 2012 the reaction pathways for the methanol synthesis at oxygen vacancies at the polar 000-1 ZnO surface. We also investigated how the doping of the zinc oxide surfaces influences the activation of molecular hydrogen and the electronic properties of the catalyst and characterized adsorbates of N2 and NO on TiO2 (rutil) surfaces. Further details are given here

In a current project within the framework of the Sino-German initiative "Novel Functional Materials for Sustainable Chemistry" we study hydrogenation reactions on carbon supported palladium clusters. The purpose of this project is to reveal the influence of support materials on the electronic properties of metal catalysts and their influence on the mechanisms and barriers of the catalyzed reactions. We want to obtain an in-depth insight into metal-support interactions and the importance of metal-support interactions for the whole process of heterogeneous catalysis. To study the complex interplay between hydrogen, metal-nanoparticles and support material, we perform state-of-the-art electronic structure calculations. One of our main interests is to study the influence of heteroatoms in the support material on the heterogeneous catalyzed hydrogenation reaction.

In summer 2018 the DFG funded the new CRC/Transregio 247 "Heterogeneous Oxidation Catalysis in the Liquid Phase – Mechanisms and Materials in Thermal, Electro-, and Photocatalysis" in which our group is involved with the project "Quantum Chemical Investigation of Catalytic Cycles on Transition Metal Oxides".


Interactions with Electromagnetic Fields: Linear and Nonlinear Optical Properties

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 + f2 · 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

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 C6 dispersion coefficients can be obtained from the frequency-dependent dipole polarizability.


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

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