This project supported by the Theoretical and Computational Chemistry program will study non-additivity of intermolecular interactions in a variety of chemical systems using first principles ab-initio quantum mechanical methods. Weak intermolecular forces and interactions are of key importance in studies of the properties of matter in condensed phases. In such an environment, the interaction in a pair of molecules affects the ways each of them interacts with a third. The assumption of pairwise additivity of intermolecular interactions is thus a very poor approximation. Most current means of simulating condensed phase reactions either disregard non-additivity of interactions entirely, or attempt to include it in a very crude manner, mixed together with a number of errors in the interaction potentials. One common misconception about non-additivity is that it originates from a single source. However, in reality it arises from a number of fundamentally different physical phenomena. This theoretical research is aimed at elucidating all of the effects responsible for non-additivity and to break these down into components which are labeled dispersion, induction and exchange. Such an analysis will lead to an improved understanding of these effects on a fundamental level. This work will result in the development of a number of analytical potentials which explicitly include many-body effects and are thus suitable for simulations of condensed phases. as well as properties of clusters.
