Modeling core excitations and preserving spin symmetry in molecular systems

Abstract

This dissertation details work in two general areas: the modeling of X-ray absorption spectroscopy (XAS) and the spin symmetry in molecular simulations. The first chapter introduces electronic structure theory (EST) techniques used in the later sections and XAS. The following chapter discusses calibration of the energy-specific time-dependent density functional theory approach for modeling core excitations and proposes appropriate functionals and basis sets. The time-resolved X-ray absorption of a nickel porphyrin system is also investigated to understand the relaxation pathway following an initial excitation. The very accurate energy-specific equation of motion coupled cluster approach is introduced next and a solution to the origin dependence problem for quadrupole-allowed transitions is investigated. The final two chapters focuses on the issue of spin symmetry in EST and uses two alternative approaches to preserve this as a symmetry of a molecular wave function. The system can be constrained to a particular spin symmetry in the single-determinant framework and then additional electron correlation can be included through the configuration interaction method. A time-dependent extension of this approach and its application to modeling linear and nonlinear electric properties is presented. Spin symmetry can also be restored to broken symmetry Slater determinants by using a projection operator. An efficient implementation of projected Hartree-Fock is presented in detail.