Junior Professorship for Theoretical Physical Chemistry of Large Molecules

 
 

Research


Our research field is theoretical chemistry and centers around the development and application of efficient simulation methods for large molecules, molecular aggregates, and condensed phase-systems. The aim is to enable the predictive simulation of large molecules and functional materials that cannot straightforwardly be treated with conventional ab initio quantum chemistry methods. For this purpose, we are particularly interested in the development of purpose-specific models that offer an optimum accuracy/cost ratio in the simulation of specific target properties. By incorporating these in multi-level and multi-scale workflow, sophisticated simulations across different time and length scales become feasible.

The development of approximate – mostly semiempirical – electronic structure and multi-scale methods are among the main directions pursued in our group. To accelerate calculation steps that represent computational bottlenecks, we make also make use of inexpensive heterogeneous computing architectures involving graphics processing units (GPUs) along with appropriately adapted algorithms. In a complementary manner, we make use of machine learning models mostly to further accelerate the simulation procedure and improve the accuracy.

The chemical problems addressed in our group range from mechanistic studies of chemical reactions and photochemical processes to spectroscopic analyses of large molecules and molecular aggregates. Furthermore, we are currently interested in the simulation and discovery of new electrolyte components. In this context, we also aim for the simulation of ion transport processes in polymer electrolyte materials. Understanding the key factors that – at a molecular level – enable fast ion diffusion will help to develop novel battery materials.

Our research is currently funded by Ministry of Culture and Science of North Rhine-Westphalia via the NRW Returning Scholars Program.

 

Electronic Structure Theory Methods for…

 


...Potential Energy Surface Exploration


In the pursuit of enabling predictive quantum chemical simulations, ideally on a regular desktop workstation, we develop efficient electronic structure theory methods that enable the exploration of the potential energy surface. Recently, the family of semiempirical extended tight-binding (xTB) methods has been developed for this purpose. The most sophisticated variant, GFN2-xTB, has become one of the most widely used semiempirical quantum mechanical (SQM) methods. It represents the first off-the-shelf tight binding method that includes anisotropic second order charge fluctuation effects and is parametrized for all elements up to radon.



C. Bannwarth, S. Ehlert, S. Grimme
GFN2-xTB – An Accurate and Broadly Parametrized Self-Consistent Tight-Binding Quantum Chemical Method with Multipole Electrostatics and Density-Dependent Dispersion Contributions,
J. Chem. Theory Comput. 2019; 15:1652–1671.
DOI: 10.1021/acs.jctc.8b01176

  method-plot Copyright: © C. Bannwarth
 
 


We are currently working on improved SQM methods. The aims are to achieve a with broader applicability and improved performance. Furthermore, we make use of consumer-grade graphics processing units (GPUs) to enable faster calculation of computational bottlenecks.

 
 
 

… Simulation of Photochemical Processes


Photochemistry is omnipresent in nature, e.g., in the vision process and essential in the conversion of the photon energy into chemical energy. Most commonly, spin-preserving photochemistry is simulated by means of non-adiabatic dynamics simulations. This poses a huge challenge on the underlying electronic structure method: the method must be highly efficient to allow the simulation of multiple molecular dynamics trajectories, it needs to incorporate static electron correlation to describe conical intersections properly, and it must include a treatment of dynamic electron correlation to obtain correct relative energies of excited states. While suitable electronic structure methods are available for most ground state processes, there is no method that satisfies all the of these requirements for excited-state chemistry.

Combinations of density functional and wave function theory offer an appealing avenue to efficiently treat dynamic and static correlation. However, no clear recipe exists on how to achieve this optimum combination. We recently presented the hole-hole Tamm-Dancoff-approximated density functional theory (hh-TDA) approach as a possible way to combine these two worlds. Here, the ground and excited states are generated by double annihilation of two electrons from an intermediate state with two extra electrons. This method has successfully been used to unravel of the wavelength-dependent photochemistry of azobenzene purely by means of ab initio electronic structure and non-adiabatic dynamics simulations.

 
  httda-merge Copyright: © C. Bannwarth
 
 

C. Bannwarth, J. K. Yu, E. G. Hohenstein, T. J. Martínez;
Hole-hole Tamm-Dancoff-approximated density functional theory: A highly efficient electronic structure method incorporating dynamic and static correlation,
J. Chem. Phys. 2020; 153:024 110.
DOI: 10.1063/5.0003985


J. K. Yu, C. Bannwarth, R. Liang, E. G. Hohenstein, T. J. Martínez;
Nonadiabatic Dynamics Simulation of the Wavelength-Dependent Photochemistry of Azobenzene Involving Excitations to the nπ* and ππ* Excited States
J. Am. Chem. Soc. 2020; 142:20680–20690.
DOI: 10.1021/jacs.0c09056

Despite the remarkable performance of hh-TDA in the simulation of low-lying excited states, it is restricted to excitations to lowest unoccupied orbital, only. We are thus interested in developing methods that keep the merits of hh-TDA but overcome the restriction of the considered active space. This will allow simulations of photochemical processes in a wider range of systems, including different light-harvesting materials and photoswitches.

 
 


...Electronic Absorption and Circular Dichroism Spectroscopy


Chiral compounds are omnipresent in nature and form the basis of life. As a result, drug and other bioactive molecules are often chiral and their synthesis is one of the main aims in the field of asymmetric synthesis. To identify the absolute configuration of chiral molecules in solution, different spectroscopic methods like electronic circular dichroism (ECD) spectropscopy can be used. Circular dichroism refers to the differential absorption of left and right circularly polarized light (see schematic below left).

 
  ecd-plot-merged Copyright: © C. Bannwarth
 
 

By comparing experimentally recorded and theoretically simulated ECD spectra, the absolute configuration of chiral molecules can be identified. Since ECD spectra are sensitive to structural changes they can also be used to study folding processes of proteins. We have worked on the simplified time-dependent density functional theory (sTD-DFT) methods that enable very fast simulation of such spectra.

C. Bannwarth, S. Grimme;
A simplified time-dependent density functional theory approach for electronic ultraviolet and circular dichroism spectra of very large molecules,
Comput. Theor. Chem. 2014; 1040–1041:45–53.
DOI: 10.1016/j.comptc.2014.02.023

S. Grimme, C. Bannwarth;
Ultra-fast computation of electronic spectra for large systems by tight-binding based simplified Tamm-Dancoff approximation (sTDA-xTB),
J. Chem. Phys. 2016; 145:054 103.
DOI: 10.1063/1.4959605



Currently, we are working on a multi-scale approach to extend the simplified excited state methods to large molecular assemblies with several thousand of atoms.