Quantum Chemistry Meets X-Ray Absorption Spectroscopy: Theory and Applications to K-Shell Excitations in Molecules
In this thesis, we present our work in pursuit of a novel method for computing core-excited states of molecules: non-orthogonal configuration interaction singles, or NOCIS. NOCIS is a black-box variant of NOCI which, when applied to K-shell excitations, uses A different core-ionized determinants for a molecule with A atoms of a given element to form single substitutions. NOCIS is a variational, spin-pure, size-consistent ab initio method that dramatically improves on standard CIS for core excitations by capturing essential orbital relaxation effects associated with the core hole, in addition to essential configuration interaction. We also describe a more computationally efficient modification of NOCIS called one-center NOCIS (1C-NOCIS) for open-shell molecules which is intermediate between NOCIS and the computationally less demanding static exchange approximation (STEX). These methods are efficiently implemented and applied in the context of various core-excited state types, namely molecules initially in singlet, doublet, and triplet states represented by a single determinant wavefunction. Finally, this thesis also presents an overview of collaborative excited-state work in various settings, including a foray into valence excited states.