Kwantumchemische methoden voor spectra en eigenschappen in de gecondenseerde fase

Computational and theoretical chemistry has become an important pillar in our modern understanding of chemistry. The usage of mathematical models based, for example, on thermodynamic, kinetic, or quantum mechanical theories aids chemical research on a daily basis. Specifically, computational methods based on quantum mechanics are incredibly important because they enable access to information such as molecular structure, reaction paths, spectroscopic data, or molecular properties. However, these methods often require one to make a trade-off between accuracy and computational effort, often making it unfeasible to model realistic systems with the most robust theories. For this reason, the development of computational protocols designed for studying systems in condensed phases, where most of the chemistry actually takes place, while still using accurate quantum mechanical methods, is an active area of research for theoretical chemists. This thesis aims to develop new computational protocols for modelling spectra and properties in condensed phase. In particular, Auger decay and electronic dipole polarizabilities were studied using the developed protocols. The proposed approaches have coupled-cluster theory as their foundation. For the description of Auger decay, coupled-cluster theory is combined with complex variable techniques based on non-Hermitian quantum mechanics, which provide a suitable framework for describing the transient nature of the metastable states involved in such processes. The computational protocols presented in this thesis streamline the modelling of Auger spectra from the results of these calculations, which was illustrated for organic molecules and heavy metals. Meanwhile, two different approaches that employ the projection-based quantum embedding method were proposed for the study of supermolecular properties of systems in condensed phase. Analytical derivatives serve as an efficient to compute molecular properties. For this reason, the implementation of first derivatives for coupled-cluster in density functional theory embedding was also addressed in this work.