UC Berkeley

Understanding excited states is vital to photochemistry and spectroscopy, yet the study of excited states from a theoretical viewpoint is often much more challenging than that of ground states due to the more complicated nature of excited states systems. This thesis is primarily focused on a series of topics regarding excited state electronic structure theory, aiming to improve the accuracy, efficiency and interpretability of computational methods. In particular, we generalize the recently developed ALMO-CIS approach, which is a formulation of configuration interaction singles (CIS) for molecular complexes or clusters based on absolutely-localized molecular orbitals (ALMOs), in two ways. The first is to reintroduce charge-transfer (CT) effects, and the resulting ALMO-CIS+CT method is shown to provide better accuracy in simulating spectra of large helium clusters. Secondly, a multi-reference version of ALMO-CIS is formulated, and shown to be a promising method for systems where pure charge-transfer excitations are important. We also develop and implement an energy decomposition analysis (EDA) method for studying intermolecular interactions in excited molecular systems. This EDA scheme brings insight to interesting excited-state phenomena such as solvatochromism, charge-transfer-to-solvent (CTTS) spectrum and the formation of excimers.