New Tools and their Application for the study of nonadiabatic processes influenced by conical intersections

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David Yarkony is supported by an award from the Theory, Models and Computational Chemistry program in the Chemistry Division to develop a novel method of treating conical intersections on potential energy surfaces. Characterization of the electronic potential energy surfaces, including conical intersections and inter-state interactions is essential to the successful simulation of nonadiabatic processes. The incorporation of conical intersections into "fit" potential energy surfaces is a challenging task. In the commonly used nonadiabatic dynamics method known as ab initio direct dynamics or dynamics on the fly, this problem is circumvented by obtaining the electronic structure information directly from ab initio electronic structure calculations as required. At present, given the intrinsically high cost of these electronic structure calculations, in ab initio nonadiabatic direct dynamics, one is routinely forced to trade off accuracy of the electronic wave functions for speed of evaluation. The approach developed in the PI's research group avoids this bottleneck, obtaining the electronic structure data from a non-local quasi-diabatic Hamiltonian Hd (the 'fit' Hamiltonian) that reliably approximates the high quality ab initio electronic structure data used to construct it. This Hd correctly locates conical intersection seams, local minima, saddle points, and dissociation asymptotes, and reproduces the local topography of a conical intersection.Nonadiabatic processes in which molecules change their electronic states radiationlessly, that is, without absorbing or emitting a photon, are intimately involved in such essential areas as energy storage, vision and photochemistry. Once routinely dismissed as an arcane theoretical notion, conical intersections, i.e. points of intersection of two (or more), electronic potential energy surfaces with the topography of a double (or higher order) cone, are now understood to play a key role in nonadiabatic processes.The Hd based electronic structure apparatus is being interfaced into existing direct dynamics procedures and made available to a broad scientific community. It is of particular interest to photoelectron spectroscopists. Photoelectron spectroscopy is a valuable tool in accessing nonadiabatic interactions in molecules. Both undergraduate and graduate students take part in this research project, gaining valuable training in developing and implementing sophosticated methods in computational chemistry.