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Abstract
Ab initio quantum chemistry is an incredibly useful tool to compute highly accurate energies, frequencies, and molecular geometries for a plethora of chemical systems, but the great computational cost associated with such accurate methods hinders the efficacy of its use without cost-reductive techniques. Perhaps one of the most prominent ab initio methods, coupled cluster theory with perturbative excitations can approximate highly accurate energies on small molecular systems while greatly decreasing the computational cost compared to coupled cluster implementations of the same order. In this dissertation, the theory behind various ab initio quantum chemical methods, including coupled cluster theory and perturbation theory, are discussed. These methods are then employed to compute the enthalpies of formation for a set of Criegee intermediates, or carbonyl oxides. Additionally, coupled cluster theory is employed on second-row homonuclear diatomic molecules to compute their potential energy surfaces and relevant spectroscopic constants. The convergence of coupled cluster methods on each of these systems is explored along with the performance of perturbative excitation methods, including the popular CCSD(T) and CCSDT(Q) methods. Through these studies, it is shown that the use of perturbative excitations in coupled cluster computations is an accurate, cost-effective approximation to more expensive methods unless applied to multi-reference systems, on which the performance can worsen significantly.