Examination and Development of the Correlation Consistent Composite Approach

The primary focus of this dissertation is the advancement of the correlation consistent composite approach (ccCA) methodology from its original formulation to the current implementation. Although for large main group test sets which contained both first- (Li-Ne) and second-row (Na-Ar) species ccCA produced chemical accuracy (generally estimated as a deviation of ~1 kcal mol-1 from reliable experiment), the second-row species were smaller in molecular size in comparison to their corresponding first-row species. Previous theoretical work has shown that the accuracy for theoretical calculations involving second-row species (specifically sulfur-containing species) are more basis set dependent than first-row species. Therefore, an analysis of the accuracy of ccCA for sulfur-containing species is warranted. The ccCA methodology is used to evaluate both enthalpies of formation and bond dissociation energies of sulfur-containing species as well as examine isomerization energies for three sets of sulfur-containing isomers. During the testing of ccCA for sulfur-containing species two observations were made which led to further investigations. First, there is no agreement between different theoretical methodologies on the lowest energetic isomer between SNO and NSO. In fact, G3 and G3B3 which differ only by the geometry of the single-point calculations do not agree on the lowest isomer. For this reason, larger, more complete theoretical treatments of SNO and NSO are investigated. Second, for open-shell sulfur-containing systems the accuracy of the ccCA methodology begins to degrade when spin-contamination becomes non-negligible. Therefore, we investigate the accuracy of the ccCA methodology when spin-contamination is removed from the wavefunction. Finally, the ccCA methodology is utilized in a multilayer ONIOM approach as the high level of theory in conjunction with density functional theory as the low level for the C-H bond dissociation energies of anthracene and fluorene analogues