• Born-Oppenheimer approximation
• electronic strong coupling
• polaritonic chemistry
• vibrational strong coupling

During the last few years, increasing attention has been paid to the possibility to alter molecular properties and reactivity using strong light-matter interactions. The theoretical description of these systems requires an exact treatment of the quantized electromagnetic field and its correlation with electrons and nuclei. In this framework, the generalization of the Born-Oppenheimer approximation is not straightforward. This thesis focuses on the different strategies to address this problem. First, the existing approaches are analyzed and compared. While the polaritonic approach allows to properly describe electron-photon correlation, Cavity Born-Oppenheimer approximation provides a good framework to describe vibrational strong coupling regimes. In the latter case, though, electronic structure methods have to be generalized in order to include field-related effects. In this thesis, Cavity Born-Oppenheimer Hartree-Fock and Coupled Cluster theory are presented. Their implementation is then applied to the study of intermolecular interactions in dimers. The analysis of the results obtained highlights the presence of spurious effects in the CBOA description. In a second part of the thesis, an alternative approach to the generalization of the Born-Oppenheimer approximation is proposed. This new parametrization aims to provide the flexibility needed to describe both electronic and vibrational strong coupling regimes. To this end, both these conditions are explored in the results section, where this method is applied to the study of small molecules. Here, the effects of electron-photon and nuclei-photon interactions are analyzed.