Solvent methods in coupled-cluster theory
Thanthiriwatte, Kanchana Sahan
AdvisorGwaltney, Steven R.
CommitteeLewis, Edwin A.
Foster, Stephen C.
This dissertation describes the implementation of the molecular electronic structure calculations with an implicit solvent model using coupled-cluster theory. The theory for and the implementation of the solvent reaction field method (SCRF) and the reference interaction site model (RISM) at the coupled-cluster singles and doubles (CCSD) are presented. In the SCRF model a solute molecule is placed in a spherical cavity, and the outer solvent is represented by a dielectric continuum, which is characterized by the dielectric constant of the solvent. The reaction field is introduced to the system by using the multipole moment expansion of the electronic structure of the solute molecule and the dielectric constant. The SCRF method has been used to calculate the conformational equilibrium and the rotational barriers of 1,2-dichloroethane in vacuum and in different solvents. The calculated results are compared with experimental values. In addition, the solvent effects on the energetics of the mechanism of nitration of benzene are reported using the implemented CCSD-SCRF model. The idea of RISM is to replace the reaction field in continuum models by a microscopic expression in terms of the site-site radial distribution functions between solute and solvent, which can be calculated from the RISM integral equations. The statistical solvent distribution around the solute is determined based on the electronic structure of the solute, while the electronic structure of solute is influenced by the surrounding solvent distribution. Therefore, the wave function and the RISM equations are solved self-consistently with CCSD. Pair correlation functions, partial atomic charges, and solvation free energies of water and N-methylacetamide are calculated in liquid water using proposed theory. Both the CC-SCRF and CC-RISM methods have been implemented in a developmental version of the Q-Chem 3.2 quantum chemistry package.