Browsing by Subject "Diabatization"

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    Electronic Structure Method Development for Excited-State Chemistry
    (2017-09) Hoyer, Chad
    The accurate modeling of photochemistry requires robust dynamics simulations on correct potential surfaces. A pragmatic approach is to first compute potential surfaces with an accurate electronic structure method, fit the surfaces to an analytic function, and then run dynamics using the fitted surfaces. This approach will be referred to as fitted dynamics. The focus of this work is on the electronic structure aspect of fitted dynamics. Specifically, I will discuss my work on benchmarking and method development of multiconfiguration pair-density functional theory (MC-PDFT) and diabatization method development. MC-PDFT is very similar to Kohn-Sham Density Functional Theory (KS-DFT); however, MC-PDFT uses a multiconfigurational (MC) wave function (WF) instead of a single Slater determinant (SD), the MC-PDFT energy is a functional of the den- sity and on-top pair density instead of only the density, and the MC-PDFT energy is computed via post-SCF instead of optimizing the molecular orbitals (MOs). Due to the MC nature of excited states, MC-PDFT is a promising alternative to KS-DFT for photochemistry. To check if MC-PDFT is useful for photochemistry, we first bench- marked it on vertical excitation energies of atoms and organic molecules. We found that MC-PDFT exhibits quantitative accuracy. We have also explored new theory developments, which may be of use for practical photochemistry applications. With regards to diabatization, the dipole-quadrupole (DQ) and dipole-quadrupole- electrostatic-potential (DQ ) diabatization schemes were developed. They diabatize using simple one-electron properties and the methods exhibit applicability to general systems.
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    Photochemical Studies: Method Development And Evaluation
    (2020-03) Parker, Kelsey
    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.