Studies of molecular systems with multiconfigurational methods

Abstract: Research in theoretical chemistry is aimed at finding computational algorithms that produce as accurate results as possible, for as large molecules as possible, with a minimum of computational effort and to apply these methods to molecular systems of chemical interest. The most accurate theoretical methods, often referred to as ab initio methods, are derived from the fundamental laws of physics and they do not rely on any empirical information. This thesis is devoted to the development of so-called multiconfigurational ab initio methods and their application to molecular systems which require such a description. Theoretical advances, within the areas of excited state geometry optimization and second order electronic transition intensities, are reported. The corresponding new implementations are applied to the geometry optimization of the 7-azaindole dimer and the lowest totally symmetric two-photon transition in trans-stilbene. The thesis also encompasses a multiconfigurational study of the spectroscopy of the C3H2 isomers and highly accurate calculations on the ammonia dimer interaction energy.