Electronic Structure Method Development for Excited-State Chemistry

Abstract

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.