The Theoretical and Computational Chemistry Program is supporting Professor Mark Cybulski at Miami University. The main goal of this research project is to implement a method for analyzing weak intermolecular interactions at higher levels of Moller-Plesset perturbation theory (MP3 and MP4) and its infinite order generalization coupled-cluster theory (more specifically, L-CCD and CCD models). The method involves the joint use of the supermolecule approach and perturbation theory of intermolecular interactions at one of the aforementioned levels of theory. The concurrent use of both approaches permits the interpretation of the supermolecule interaction energies in terms of rigorously evaluated electrostatic, exchange, induction and dispersion energies. Specific applications will focus on complexes with unusually large higher-order correlation effects such as Ar-HCN and NH3-CO. In addition, the magnitude of the three-body nonadditive effects in an open shell complex Ar2-CH will be examined. In parallel with calculations of interaction energies, calculations of molecular properties of the complexes will be performed and the influence of component energies on molecular properties such as dipole moments and nuclear quadrupole coupling constants will be carefully examined. There is a growing interest in understanding weak intermolecular interactions in biological systems. The interactions are considered `weak` in the sense that the interacting species retain, to a large degree, their individual character. Intermolecular interactions are responsible for base-pair formation across the strands of DNA and RNA. Unfortunately, the information about the energetics of interaction is not, in general, accessible experimentally. The most promising approach for studying the energetics of intermolecular interactions is computational chemistry. The goal of this project is to further develop a method for studying intermolecular interactions which proves particularly useful in providing insight into the nature of weak interactions. Applications will, initially, focus on smaller complexes such as NH3-CO and Ar-HCN, in which interactions were found to be difficult to interpret.