We study in this thesis intermolecular interactions through a localized orbitals point of view. This concerns on one hand the intermolecular interactions generated by the occupied localized orbitals such as the electrostatic interaction, and on the other hand the intermolecular interactions like the dispersion interaction that involve virtual orbitals. We evaluate in a first part the electrostatic interactions on frozen monomer charge densities provided by multipolar distributions on occupied localized orbitals. These give chemically meaningful representations of molecular charge densities. We show that multipolar distributions on localized orbitals are suitable to describe intermolecular electrostatic interactions if the interaction is truncated to a suitable order, making localized orbitals potentially interesting to model electrostatic interactions in a force field. We use then the properties of a priori constructed localized orbitals on a complex in order to define a reference multipolar interaction for the electrostatic interaction of relaxed charge densities. We evaluate hence the capacity of multipolar distributions based on relaxed localized orbitals to decribe the relaxed electrostatic interaction. In a second part, localizing both occupied and virtual orbitals in an intermolecular framework allows us to attribute the orbitals of a noncovalent system to each of its fragments (and to divide the excitations by classes), and select in a further step only the most relevant excitations to the intermolecular post-Hartree-Fock correlation energy. We propose two different methods we developped in this work to select the excitations in the general framework of the range separated density functional theory (DFT) coupled to the random phase approximation (RPA). The first method is based on a simple energetic criterion while the other is based on a selection of only one class, namely the dispersion-type excitations. We show afterwards both the usefulness and the limits of those two methods of selection on complexes with various types of interaction.
