CAREER: Theoretical and computational methods for spin-forbidden processes in complex systems

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Sergey Varganov of the University of Nevada, Reno is supported by an award from the Chemical Theory, Models, and Computational Methods program in the Chemistry Division to develop new theoretical and computational methods for studying chemical reactions involving a change of electron spin. (Electrons can be thought of as tiny spinning tops, but in reality "electron spin" is a purely quantum mechanical property that has no classical physical analogue.) Such reactions, referred to as "spin-forbidden", are important in different areas of chemistry, physics and biology. Varganov and his research group focus on two types of methods: a highly accurate approach applicable to small molecules and a simple statistical approach designed for very large biological molecules. They are using these methods to understand the fundamental properties of spin-forbidden reactions and the role of electron spin in complex biological molecules capable of accelerating industrially important reactions, such as hydrogen reduction and oxidation. These studies are facilitating development of new advanced materials for different energy applications. Prof. Varganov is also developing demonstration and computational tools to enhance the teaching of chemistry and the impact of outreach activities. These tools include 3D-printed models representing energy landscapes of chemical reactions and simplified versions of computational chemistry methods suitable for use in graduate and undergraduate chemistry courses. This project is aimed at developing state of the art theoretical and computational methods to investigate the kinetics and dynamics of spin-forbidden processes, including intersystem crossings, spin crossovers and spin-forbidden reactions, in complex systems. The focus is on the novel nonadiabatic statistical theory and efficient multiple spawning molecular dynamics methods. The statistical theory is made applicable to spin-forbidden processes in systems with thousands of atoms by implementing new algorithms within the fragment molecular orbital method. The multiple spawning molecular dynamics provides a general approach to account for dynamic effects in the spin-forbidden kinetics calculations. The new methods are validated on small molecules and used to investigate catalytic hydrogen oxidation/reduction on the metalloenzymes [NiFe]-hydrogenase and Ni-substituted rubredoxin. The education component is centered on the molecular dynamics demonstrations using 3D-printed potential energy surfaces of simple chemical reactions.