Complex molecular systems play a key role in many areas of physics and chemistry. They serve a multitude of purposes in various applications such as homogeneous inorganic and bioinorganic catalysts or modern materials. Understanding the physical and chemical properties is the first step on the way to new and improved catalysts and materials.
The focus of our research group lies in the development and application of novel quantum methods to investigate complex molecular systems on the level of quantum theory. Our methods are mostly based on the “Density Matrix Renormalization Group” which has found its way into the field of quantum chemistry in the last decade. It allows for accurate “first principles” calculations on large molecules that are out of reach for comparable standard quantum chemical methods. The focus of our applied studies are the unique electronic properties of polynuclear transition metal clusters and excited states of large organic compounds.
The research group is affiliated to the Max-Planck Institut für Kohlenforschung in Muelheim an der Ruhr and is located at the Ruhr-University Bochum. It is funded by the Max Planck society in the framework of the “Otto-Hahn Award” program.
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The theory development in our research group will be centered around the “Density Matrix Renormalization Group” which was introduced to quantum chemistry in the early 2000's. Our contributions to the field are mostly concerned with a) the inclusion of dynamic electron correlation to molecular DMRG calculations and b) molecular DMRG calculations beyond the Born-Oppenheimer approximation.
a) Molecular DMRG calculations as they are now performed regularly can be regarded as an approximation of complete active space configuration interaction (CAS-CI) or complete active space self-consistent field (CAS-SCF) calculations. These calculations cover static correlation effects but neglect most of the dynamic electron correlation. Inclusion of the missing dynamic electron correlation using perturbation theory is a current field of work in our research group where we collaborate with the group of Prof. Garnet Chan at Princeton University.
b) When two or more adiabatic potential energy surfaces cross in the course of a chemical reaction during relaxation processes the system can change its quantum state. As a result of the switch between potential energy surfaces, the system's electronic properties can change dramatically. The dynamics of this process are governed by the strength of non-adiabatic and/or spin-orbit coupling effects. We develop methods to evaluate non-adiabatic and spin-orbit coupling effects in the framework of molecular DMRG theory.
Many inorganic and bioinorganic polynuclear transition metal clusters comprise extraordinary physical and chemical properties. For example, some of these compounds are able to catalyze chemical reactions that would otherwise be inaccessible or energetically unfavorable. The activation of N2 or the splitting of water into hydrogen and oxygen are prominent examples. It is believed that one of the key factors that lead to the unique reactivity of many polynuclear transition metal clusters is the abundance of low-lying electronic states. The ability of many transition metals to easily change their oxidation state allows for relocation of charge in a system with more than one metal center at relatively low energy cost. Furthermore, the number of low-lying states is greatly increased by the different possibilities to couple the local spins at the metal centers to a total spin. Our goal is to investigate the role of these low-lying excited states on the physical properties and the reactivity of polynuclear transition metal clusters.
Electronically excited states play an important role in many areas of chemistry, e.g. photochemistry. However, the quantum chemical description of electronically excited states is complicated and computationally demanding as the system size grows. Our advanced molecular DMRG methodology is well suited for the challenges that arise from the calculation of electronically excited states of medium sized (~50-100 atoms) organic compounds. It provides the required complexity of the excited wavefunction and is efficiently programmed. Our current interest is to investigate the excited states of medium-sized acenes which are applied in optical electronic devices.
Dr. Roemelt, Michael