GAUSSIAN MULTIPOLES IN PRACTICE - ELECTROSTATIC ENERGIES FOR INTERMOLECULAR POTENTIALS

A method is presented for calculating the total electrostatic interact ion energies between molecules from ab initio monomer wave functions. This approach differs from existing methods, such as Stone's distribut ed multipole analysis (DMA), in including the short-range penetration energy as well as the long-range multipolar energy. The monomer charge densities are expressed as distributed series of atom-centered functi ons which we call Gaussian multipoles; these are analogous to the dist ributed point multipoles used in DMA. Our procedure has been encoded i n the GMUL program. Calculations have been performed on the formamide/ formaldehyde complex, a model system for N-H ... O hydrogen bonding in biological molecules, and also on guanidinium/benzene, modeling amino /aromatic interactions in proteins. We find that the penetration energ y can be significant, especially in its contribution to the variation of the electrostatic energy with interaction geometry. A hybrid method , which uses Gaussian multipoles for short-range atom pair interaction s and point multipoles for long-range ones, allows the electrostatic e nergies, including penetration, to be calculated at a much reduced cos t. We also note that the penetration energy may provide the best route to an atom-atom anisotropic model for the exchange-repulsion energy i n intermolecular potentials. (C) 1994 by John Wiley and Sons, Inc.