Multilevel coupled cluster theory: Development, implementation and application of multilevel coupled cluster theory for core and valence excited states

Abstract

This thesis consist of seven papers and an introduction describing the ongoing development and applications of multilevel coupled cluster theory. Coupled cluster theory is one of the most accurate, but also one of the most computationally demanding methods currently available in electronic structure theory. Multilevel coupled cluster theory can greatly reduce the cost by treating only a part of the molecule with a high level theory and the rest more approximately. X-ray spectroscopy involving core excitations is becoming increasingly important in chemical investigations. Modelling core excitations is challenging and require high level methods. This thesis demonstrates how coupled cluster models can be combined with novel experimental techniques to follow the evolution of excited states in thymine. Core excitations are highly local, being dominated by a single core electron. This makes them well suited for the multilevel coupled cluster approach. The method can be combined with a new type of orbitals, generated specifically for the excitation being investigated. We achieved almost two orders of magnitude reductions in computational cost with this approach.