Coupled cluster methods for nonadiabatic molecular dynamics

Abstract

Reliably predicting nuclear dynamics in excited electronic states requires an accurate representation of the involved electronic states. One challenge in this respect is the correct description of electronic degeneracies, points where two or more electronic states have the same energy. In particular, standard coupled cluster methods fail to give a physical description of such excited state conical intersections. As a consequence, coupled cluster theory has not been successfully applied in predicting excited state dynamics. In this thesis, we give a new analysis of the problem and introduce a modified method, called similarity constrained coupled cluster theory, which is shown to be capable of giving a correct description of conical intersections between excited states. The nuclear Schrödinger equations in coupled cluster theory are also presented and analyzed. In addition, algorithms for improved efficiency are introduced, in particular, a new Cholesky decomposition algorithm for the electron repulsion integrals and multimodel Newton algorithms for converging coupled cluster equations.