Coupled cluster investigations of spectroscopic properties in molecular and crystalline systems

Abstract

Theoretical studies of spectroscopic properties are of great value, both by themselves and in combination with experiments. The highly accurate coupled cluster methods are attractive options for such studies. Due to their high polynomial scaling, coupled cluster methods are limited by the system size which can be treated, and also run into issues when problematic degeneracies occur. The thesis is concerned with how coupled cluster can be used to obtain spectroscopic properties of systems exhibiting such problematic attributes, specifically photoswitches and molecular crystals. Most photoswitch studies are conducted using multireference methods due to the presence of degenerate states. However, we show that CC3 can generate potential energy surfaces in good agreement with high-level multireference results. Therefore, coupled cluster could constitute a valuable tool for investigating photoswitches. In the case of molecular crystals, which are “infinite” systems, we present a finite cluster approach for obtaining properties of a local region. This approach highlights a currently underexploited feature and makes it possible to use coupled cluster methods for the study of local spectroscopic properties. The approach is used to elucidate the potential of X-rays absorption spectroscopy for the study of methane and carbon-dioxide clathrate. Methane clathrate is a potential energy source, while carbon-dioxide clathrate could be used in carbon capture and storage.