Parker, Kelsey2020-08-252020-08-252020-03https://hdl.handle.net/11299/215068University of Minnesota Ph.D. dissertation. March 2020. Major: Chemistry. Advisor: Donald Truhlar. 1 computer file (PDF); xiv, 162 pages.Photochemistry is an important area of research, but modeling photochemical systems is complex and expensive. In this dissertation, I present my work on the testing and development of methods for photochemistry studies. Chapter 1 introduces some important concepts. In Chapter 2, I present an electronic structure method for excited states called the dual-functional Tamm-Dancoff approximation. This method is based on the relatively inexpensive time-dependent density functional theory (TDDFT) and it gives similar results to TDDFT away from conical intersection seams (CISs). Near CISs involving the ground state, DF-TDA shows an improvement over TDDFT because it gives the correct (F-2)-dimensionality of these seams, where F is the number of degrees of freedom of the potential energy surfaces. In Chapters 3 and 4, I present work on two diabatization methods for coupled electronic states: the dipole, quadrupole, electrostatic potential method and the N/D method. Neither method requires a user to define diabatic molecular orbitals, and both solve for diabatic energies without relying on following a path through coordinate space. Both methods are shown to be successful for a wide range of test cases. In Chapter 5, I present work on a method called extended Hamiltonian molecular dynamics, which is designed to be an inexpensive way to cut back on zero point energy leakage. I present our findings that this method is successful for several small test systems. Finally in Chapter 6, I present work on the construction of potential energy surfaces suitable for studying the photodissociation of methylamine. This work involves diabatization and a method called anchor points reactive potential, which is a multiscale method designed for making analytic representations of high-dimensional potential energy surfaces. My work on methylamine involves the extension of this method to a more complex system than it has previously been applied to, and I compare my surfaces to previous theoretical and experimental results and find good agreement. A theme of all this work is improving our understanding of photochemistry and designing methods to model these systems that are cost effective and generally applicable.enDiabatizationMethylaminePhotochemistryPotential Energy SurfacesThesis or Dissertation