In this project in the Theoretical and Computational Chemistry program of the Division of Chemistry the nature of nonadditivity of energies in a variety of systems is being explored by quantum mechanical methods. Both the supermolecule method and symmetry-adapted perturbation theory are being employed. A wide variety of types of systems are being studied, including nonpolar systems, polar systems, and mixed polar-nonpolar systems (such as rare gas - water aggregates). Analytic potentials are being obtained which explicitly include multibody effects and which are thus suitable for simulations of the behavior of both condensed phases and clusters. %%% The energies of very large molecules and solids are often assumed to be approximated well by the sum of the energies of the smaller molecules, or even the atoms, making up the system. In this project theoretical methods are being used to assess the accuracy of such assumptions and to develop more accurate energies of large systems.
