Diabatic Representations:New Tools, New Uses and their Application to the Study of Nonadiabatic Processes Influenced by Conical Intersections

Johns Hopkins University, Baltimore MD

Investigators

Abstract

David Yarkony of Johns Hopkins University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to develop new theoretical tools to study chemistry that arises after a molecule's electrons are excited by light energy from a laser, the sun or another source. The use of light energy to do chemistry in solar energy conversion, photosynthesis, vision and even the stability of human DNA against ultraviolet radiation all involve what scientists call electronically nonadiabatic processes, nonadiabaticity for short. Such processes involve the intersection of two different potential energy surfaces representing different electronic states. They are essentially quantum mechanical. While existing computational tools allow the description of nonadiabatic processes for smaller chemical systems, methods for treating larger, more complex systems with highly accurate theoretical approaches are still lacking. Yarkony and his coworkers are implementing techniques designed to extend the range of accurate descriptions to larger, more complex systems. These techniques and their products, are freely available to the scientific community. Perhaps ultimately even more significant than these immediate benefits, the proposed work has the potential to transform the way chemical physicists think about nonadiabaticity and compute its effects. Yarkony has introduced a reformulation of the mathematical description of nonadiabaticity by exploiting a seminal notion introduced almost 40 years ago, with the scientifically apt, but otherwise arcane appellation, the Molecular Aharonov Bohm effect. These more precise studies of nonadiabatic dynamics leading to photo-stability impact the broader societal goal of understanding societally relevant processes, such as how DNA protects itself against radiation damage. The project has three goals: (a) to develop a generally applicable procedure to incorporate the geometric phase into the single state Hamiltonian governing adiabatic dynamics enabling determination of the prevalence and magnitude of the Molecular Aharonov Bohm effect, the effect of the geometric phase attributable to energetically inaccessible conical intersections on single state nuclear dynamics; (b) to construct from an existing algorithm and new tools, a general program for determining diabatic representations for multi-state systems which incorporates in a general manner properties based diabatizations within a framework which explicitly incorporates derivative couplings and tests for spurious (also termed diabolical) singularities that can invalidate the property based diabatizations; and (c) to use existing code and the program developed in (b) to construct coupled diabatic state representations from accurate electronic structure data to describe dissociative photochemistry in bio-related systems, including initially aniline and imidazole. . The freely available programs and high quality diabatic Hamiltonians enable chemical physicists to use these deliverables to design research efforts to make further progress in nonadiabatic chemistry. A github domain provides free access to the published diabatic representations.