Perturbation theory in quantum chemistry

Abstract

This thesis is chiefly concerned with the development of cost-effective approximations to established quantum chemistry methods in terms of perturbative corrections. The central idea involves the use of low-quality calculations as a spring-board to their high-quality, but computationally demanding counterparts. The idea is applied to Self-Consistent Field and second-order M{u00F8}ller-Plesset Perturbation Theory calculations and the performance is thoroughly assessed. Results indicate that, because remarkable agreement with exact calculations can be obtained at drastically reduced cost, these perturbative corrections offer an attractive alternative to exact calculations and allow larger systems to be studied. In addition, the thesis describes two extensions of perturbative corrections in the context of orbital diagnostics and dual-basis counterpoise corrections, which both have useful practical utilities.