QM/classical multireference approaches for the study of molecules in solution

This Thesis presents a multiscale model for the study of multireference systems in complex environments, integrating the Multi-Configurational Self-Consistent Field (MCSCF) approach with the classical Fluctuating Charges (FQ) force field. Specifically, a novel set of equations for the MCSCF/FQ analytical nuclear gradient are derived to extend this method to the calculation of structural and spectroscopic properties. The Thesis provides a comprehensive overview of the theoretical foundation, implementation in the OpenMolcas software, validation through computational tests, and the application of this model to calculate vibronic spectra of aromatic molecules in solution.