Wanyi Jiang

Date of Award

4-19-2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Mark R. Hoffmann

Abstract

An efficient configuration-driven algorithm that combines the efficient unitary group approach (UGA) and the flexible macroconfiguration approach, as previously implemented in multireference configuration interaction including single and double excitations (MRCISD), has been used to newly developed methods and to essentially improve the computational efficiency of some previously developed methods. New implementations of generalized van Vleck perturbation theory (GVVPT), especially the second-order variant (GVVPT2), have shown much better performance than the previous Table-CI based codes. Incorporation of the configuration-driven MRCISD (CFGCI) program into multi-configuration self-consistent field theory (MCSCF) codes practically eliminates computational bottlenecks in CI steps. A new effective Hamiltonian-based iterative method that perturbatively includes the effects of triple and quadruple excitations into MRCISD for simultaneous consideration of multiple states, referred to as nR-MRCISD(TQ), was developed. In test calculations on benchmark molecules (H2O, C2, and CH2+), the energy deviations of nR-MRCISD(TQ) relative to full CI were found to be comparable to those of variational MRCISDTQ. This method has been demonstrated to be capable of describing both nondegenerate and strongly quasidegenerate states quite accurately. Symbolic orbitals were used to represent external orbitals in the adaptation of MRCISD to nR-MRCISD(TQ) so that the GUGA routines can be utilized directly without introducing any complex formalisms related to triple and quadruple excitations with the external orbital space. A new spin-orbit CI (SOCI) program for the model space has been developed with the spin-orbit coupling (SOC) terms being obtained by a modified Paldus and Boyle formalism. A real SO Hamiltonian matrix is constructed over “real spherical” spin functions. One-electron SO integrals are evaluated using the same recursive Obara-Saika scheme previously implemented in UNDMOL.

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