Qualitative change of character of dispersive interaction with intermolecular distance

The dispersive interaction between molecules results from Coulomb-correlated fluctuations of electrons and for large intermolecular distances it can be related to the molecular polarizabilities as in London's theory (F. London, Trans. Faraday Soc. 33, 8-26 (1937)). Here, we investigate the interaction between molecules with anisotropic polarizabilities at arbitrary distances using symmetry adapted perturbation theory, which allows us to analyze the different parts of the intermolecular potential separately. Whereas at large distances, in accordance with London's theory, there is no way to describe the dispersive interaction by a sum over pairwise isotropic atom-centered energy terms, at short distances such a description becomes possible. This surprising result has consequences for the development of molecular mechanics force fields, supports the dispersion energy terms applied in dispersion corrected density functional theory, and indicates that there is a qualitative change in electron correlation with distance. Apparently, at short distances intermolecular electron correlation is less influenced by intramolecular electron delocalization. (C) 2013 AIP Publishing LLC.