Hiking the valleys of quantum chemistry

This dissertation focuses on the exploration of minima on the potential energy surface and on more efficient ways to calculate the slope of the potential energy surface.; The electronic structure and magnetic properties of homodinuclear titanium(III) molecules with halide, hydride, and other ligands have been studied using single and multi-reference methods with triple-zeta plus polarization basis sets. Natural orbital occupation numbers suggest that the singlet states are essentially diradical in character. Dynamic electron correlation is required for calculating quantitatively accurate energy gaps between the singlet and triplet states. Isotropic interaction parameters are calculated and some of the compounds studied are predicted to be ferromagnetic at the MRMP2/TZV(p) level of theory. Zero-field splitting parameters are determined using spin-orbit coupling obtained from complete active space (CAS) SCF and multiconfigurational quasidegenerate perturbation theory (MCQDPT) wave functions. Three Breit-Pauli-based spin coupling methods were employed: Full Breit-Pauli (HSO2), the partial two-electron method (P2E), and the semi-empirical one-electron method (HSO1). Timings for these three methods are compared.; Microsolvation and combined microsolvation-continuum approaches are employed in order to examine the solvation of glycine. Glycine(H2O) n supermolecular clusters with n = 1--7 are studied with effective fragment potentials, Hartree-Fock, second-order perturbation theory, and the polarizable continuum model in order to determine the structures and relative energies of nonionized and zwitterionic glycine clusters. Bridging structures are predicted to be the global minima after 3--5 discrete water molecules are included in the calculations. Second-order perturbation theory stabilizes the zwitterionic structures by about 8--9 kcal/mol relative to the nonionized structures regardless of the number of discrete water molecules considered. Continuum calculations stabilize the zwitterionic structures relative to nonionized structures, and this effect decreases as the number of discrete water molecules is increased.; Scalable distributed data parallel analytic gradient algorithms for UMP2 and ZAPT2 perturbation theory are presented. The analytic gradient expression for second-order Z-averaged perturbation theory is revised. The Distributed Data Interface is used to access molecular orbital integral arrays stored in distributed memory. The algorithm is designed to maximize the use of local data and reduce communication costs. Several illustrative timing examples are discussed. Speedups and parallel efficiencies are reported.