On the additivity of van der Waals radii

A review of gas-phase spectroscopic data of van der Waals complexes shows that heteroatomic contacts are systematically longer and their energies lower than the averages for corresponding homoatomic ones. The differences depend on electronic polarisabilities a of contacting atoms, according to empirical formulae Delta r(w) = c[(a(2) - a(1))/a(1)](2/3) (a(1) < a(2)) and Delta E = a(a(2) - a(1))(n) (n = 1-1.2). A novel system of van der Waals radii, corrected for this effect, is suggested for rare gases, H, N, O, halogens and metals; for di- and poly-atomic molecules as well as for isolated atoms.