Dispersion energies for small organic molecules: first row atoms

Accurate atom-atom dispersion coefficients are needed if we are to be able to successfully model the intermolecular interactions between organic molecules. To go beyond the usual C-6 description we have to resort to accurate ab initio methods, and we use the Williams-Stone-Misquitta methodology to obtain anisotropic distributed dispersion coefficients to C-12 on each atomic site. Comparisons with SAPT(DFT) energies show that the resulting descriptions are very accurate, even for molecular contacts, where the asymptotic series is thought to be invalid. The complexity of our most accurate models limits their applicability, so we explore simplifications that are more suitable for use in calculations of the condensed phase, in particular, we describe a means of calculating optimized isotropic C-6 models that can be readily used in conventional programs.