Dr. Michael Römelt
Lehrstuhl für Theoretische Chemie
Phone: +49 (0)234 32 26749
Complex molecular systems, such as mono- and polynuclear transition metal compounds, play a key role in many areas of chemistry. They serve a multitude of purposes in various applications in homogeneous inorganic and bioinorganic catalysis as well as functional materials. Yet understanding and predicting their properties as well as their reactivity is a great challenge and one of the current frontiers of theoretical chemistry. Our research group focuses on both, the development of novel quantum chemical methods that are especially designed to tackle complex molecules and on the application of existing methods to tackle interesting chemical problems. The former aspect of our work is centered around modern multireference methods such as the density matrix renormalization group. With the help of these methods it is possible to correctly describe complex molecules that are difficult if not impossible to access using conventional methods like the complete active space self-consistent field (CASSCF). In addition to our efforts in theory development we conduct computational studies of different inorganic and organic systems using a variety of quantum chemical methods, ranging from density functional theory to high-level ab initio multireference methods. These studies concern chemical reactivities as well as spectroscopic properties. A short description of some of our recent research projects can be found below.
Although our research group is located at the Ruhr-University Bochum and receives funding from an Emmy-Noether fellowship we are also affiliated to the Max-Planck Institute for Coal Research in Mülheim an der Ruhr through the “Otto-Hahn Award” program. If you are interested in our research, please contact us via firstname.lastname@example.org.
In the last decade the ab initio density matrix renormalization group (DMRG) has been shown to provide a reasonable and accurate alternative to complete active space (CAS) methods as basis for molecular multireference calculations. It can be regarded as an approximation to the exact diagonalization of the large Hamiltonian matrix in the basis of many-electron wavefunctions within the active orbital space. A great advantage of DMRG is that it approximately solves a problem whose complexity scales exponentially with increasing system size by optimizing only a polynomial number of parameters. Owing to this favorable behavior DMRG is able to treat large active spaces on the order of 20–80 orbitals. However, quantitative accuracy is only reached if dynamic electron correlation effects are considered, too. Therefore we have developed a novel approach to the combination of DMRG and strongly contracted second order N-electron valence perturbation theory (SC-NEVPT2) for quantum chemical multireference calculations. The main objective of this approach is to lower the cost to treat systems with large active spaces and large orbital spaces with a moderate and controllable accuracy. As demonstrated for a dimeric Cu cluster the total energy (DMRG + SC-NEVPT2) converges rapidly and smoothly towards the exact value with increasing bond dimension which is a user-defined accuracy parameter. However, the existing method considerably suffers from the comparably high bond dimensions required to achieve chemical accuracy. In this regard, the most promising route forward for the future may be to sacrifice some of the constraints used in the projection approximation associated with the formally low scaling.
M. Roemelt, S. Guo, G. K.-L. Chan J. Chem. Phys. 2016, 144, 204113
Furthermore, we have developed an approach to describe spin-orbit coupling (SOC) on top of a regular Born-Oppenheimer DMRG calculation in the framework of quasi-degenerate perturbation theory (QDPT). This approach accounts for SOC effects on the many-electron level and can thus be thought of as the molecular equivalent of atomic Russell-Saunders or LS coupling. With the spin-orbit coupled wavefunctions at hand the molecular g-tensors can be calculated in a rigororous and phenomenological way as proposed by Gerloch and McMeeking in 1975. Importantly, since the SOC matrix is fully diagonalized within a finite set of many-electron states, our approach is able to produce qualitatively and quantitatively correct results even for systems with a near-degenerate ground state. For example, the behavior of the molecular g-values of a Mo(III) trisamidoamine catalyst as it is distorted along its Jahn-Teller axis is correctly reproduced. In contrast, regular linear-response type or single reference perturbation theory methods are bound to fail in these cases. Our method will serve to investigate the magnetic properties of complex molecular systems that cannot be properly treated by conventional DFT or CASSCF based approaches. Moreover, future developments might concern the inclusion of SOC to excited states outside the active space regime and the combination with schemes to incorporate dynamic electron correlation.
M. Roemelt, J. Chem. Phys. 2015, 143, 044112
The accurate description of magnetic level energetics in oligonuclear exchange-coupled transition-metal complexes remains a formidable challenge for quantum chemistry. The density matrix renormalization group (DMRG) brings such systems for the first time easily within reach of multireference wave function methods by enabling the use of unprecedentedly large active spaces. But does this guarantee systematic improvement in predictive ability and, if so, under which conditions? We identify operational parameters in the use of DMRG using as a test system an experimentally characterized mixed-valence bis-μ-oxo/μ-acetato Mn(III,IV) dimer, a model for the oxygen-evolving complex of photosystem II. A complete active space of all metal 3d and bridge 2p orbitals proved to be the smallest meaningful starting point; this is readily accessible with DMRG and greatly improves on the unrealistic metal-only configuration interaction or complete active space self-consistent field (CASSCF) values. Orbital optimization is critical for stabilizing the antiferromagnetic state, while a state-averaged approach over all spin states involved is required to avoid artificial deviations from isotropic behavior that are associated with state-specific calculations. Selective inclusion of localized orbital subspaces enables probing the relative contributions of different ligands and distinct superexchange pathways. Overall, however, full-valence DMRG-CASSCF calculations fall short of providing a quantitative description of the exchange coupling owing to insufficient recovery of dynamic correlation. Quantitatively accurate results can be achieved through a DMRG implementation of second order N-electron valence perturbation theory (NEVPT2) in conjunction with a full-valence metal and ligand active space. Perspectives for future applications of DMRG-CASSCF/NEVPT2 to exchange coupling in oligonuclear clusters are discussed.
M. Roemelt, V. Krewald, D. Pantazis J. Chem. Theory Comput. 2018, 14, 166-179
Among the numerous methods for the activation of carbon dioxide and its use as C1-building block, its electrocatalytic reduction constitutes a promising approach. In particular, the electrogeneration of carbon monoxide is attractive, since CO is used as a key feedstock in industry for a wide range of organic transformations. Due to the high overpotential and a variety of possible reaction pathways, a selective electroreduction of CO2 to CO is not trivial. A general possibility to overcome the issue of kinetic inhibition of an electrochemical reaction is the use of homogeneous redox-active catalysts in an indirect electrolysis. Under given conditions (substrate concentration, electrode material, electrolyte, etc.), the employment of such redox catalysts can lower kinetic barriers and often leads to an increased and/or entirely different selectivity. Recently, the groups of Dr. Robert Francke and Prof. Dr. Matthias Beller identified a cyclopentadienone iron(0) complexes as molecular catalysts for the electroreduction of CO2. It effectively converts CO2 into CO with two electrons and two protons to yield H2O. Interestingly, the reaction occurs without the addition of any acid. With the help of our quantum chemical computations it was possible to propose a reasonable reaction mechanism that it is in good agreement with the experimental results. According to our results, binding of CO2 to the catalyst in the desired η1 fashion via its C-atom can only be observed after the catalyst has been twofold reduced. Furthermore, protonation of the catalyst prior to CO2 binding facilitates the binding process through formation of a hydrogen bond. Although the highest calculated barrier is associated to the breaking of the CO bond and the simultaneous formation of an OH bond, recent experimental findings indicate that the effectiveness of the reaction is controlled by a dimerization equilibrium of a crucial intermediate. Further experimental and computational studies on this system are currently conducted in the labs of Dr. Francke, Prof. Dr. Beller and Prof. Dr. Ludwig at the University of Rostock and the Leibniz Institute for Catalysis as well as our group.
A. Rosas-Hernández, H. Junge, M. Beller, M. Roemelt, R. Francke “Cyclopentadienone iron complexes as efficient and selective catalysts for the electroreduction of CO2” Cat. Sci. Technol. 2017, 7, 459-467