The Theoretical and Computational Chemistry Program is supporting Prof. Yarkony at Johns Hopkins University over the next three years. Yarkony's research will focus on fundamental questions concerning the electronic structure aspects of electronically non-adiabatic processes. Of particular interest will be the locus of points of the conical intersection, particularly of two states of the same symmetry. The prevalence and impact of conical intersections will be studied in the ensuing grant period to investigate their role in electronically non-adiabatic processes. A second basic question concerns the diabatic bases routinely used to study the dynamics of non-adiabatic processes. Although routinely used, diabatic bases exist only in an approximate sense. Line integrals of the derivative coupling can be used to determine the size of the nonremovable part of the derivative coupling, that is the portion of the derivative coupling that must remain even in the 'most diabatic' basis. Yarkony will use this technique to identify systems for which the nonremovable part of the derivative coupling is appreciable. For these systems he will use the ab initio derivative couplings to construct the 'most diabatic' basis and compare the results with commonly used but necessarily approximate diabatization procedures. This work is expected to provide fundamental insights to the nature of diabatic bases. The chemistry of molecules exposed to light, photochemistry, is fundamentally different from the chemistry of ground state molecules. The essential difference lies in the interconversion of electronic energy created by the photoexcitation process into vibrational, rotational and translational energy as the molecule relaxes non-adiabatically to the ground electronic state. Although non-adiabatic processes are ubiquitous, occurring for example in vision, photosynthesis, and stratospheric ozone degradation, many basic questions concerning non-adiabatic processes need to be addressed. One of the principal drivers of non-adiabatic processes is a topological feature known as a conical intersection. Once thought to be rare occurrences attributable to special, symmetry, properties of a molecule, it is now clear that like non-adiabatic processes themselves conical intersections are ubiquitous. Yarkony will use a unique system of computer algorithms developed at Johns Hopkins University over the last decade to study fundamental issues related to conical intersections and the way they influence non-adiabatic processes.
