FIRST-ORDER COULOMB INTERACTION ENERGIES FOR ATOMS AND DIATOMIC-MOLECULES

Gaussian multipoles (differentiated Gaussian functions) are used to de scribe molecular charge densities. Their use simplifies considerably t he calculation of the first-order Coulomb interaction energy between p airs of molecules, which is an important part of the intermolecular po tential. Methods are developed for expressing the charge densities of linear molecules economically and accurately using Gaussian multipoles . Partial-wave expansions are calculated for the first-order Coulomb i nteraction energies of H-2-He, N-2-He, CO-He, H-2-H-2, N-2-N-2 and CO- H-2, and their convergence is discussed as a function of molecular ani sotropy and intermolecular separation. It is confirmed that the multip ole approximation is very inaccurate for physically important dimer se parations.