UC Berkeley

In this thesis, new ways of generating orbitals to accelerate quantum mechanical calculations or gain chemical insight from such calculations are presented. While antibonding orbitals are integral to our understanding of chemistry, finding them is not trivial because they cannot be uniquely defined. A new way to find them is demonstrated along with many uses they have in valence space methods such as complete active space self-consistent field (CASSCF) theory. New molecule-adapted atomic orbitals can also be obtained from the antibonding orbitals which is used for atomic charge analysis. Furthermore, new oxidation state localized orbitals (OSLOs) for assigning oxidation states are also presented and assessed across a wide range of systems to demonstrate improved performance on challenging systems. Lastly, an approach to decompose the intermolecular forces based on the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA) is described. This force decomposition analysis (FDA) separates different physically distinct contributions (such as those from permanent electrostatics and Pauli repulsion, those from the effect of induced electrostatics, and those from charge transfer or dative interactions). A series of chemical examples ranging from ion water interactions to activation of carbon dioxide by gold (and silver) anions are studied to explore the chemical insight that can be gained. The FDA may also be useful for the development and testing of molecular mechanics force fields. As an example, we use force decomposition to further validate the promising polarizable MB-UCB force field for water.