Pairwise intermolecular interaction energies between molecules of arbitrary symmetry: rotational averages in the weak interaction limit

Abstract

The fully expanded expression for the Boltzmann-averaged pairwise intermolecular interaction energy of two molecules B and C of arbitrary symmetry is determined in the weak interaction limit (<kT), including all electrostatic, induction and dispersion interactions having an r^-n dependence (where r is the B-C separation) for which n < 11. The method exploits the 'point moment function' (PMF) approach in which the Boltzmann-averaged energy is shown to reduce to a sum of terms, each containing a product of a PMF on B, and one on C. The point moment functions may be obtained in a general analytic form by averaging an arbitrary multipolar product for the isolated molecule. Two types of PMF (Δ, point chirality function, and Λ, point achirality function) are identified, leading to a natural division of contributions to the intermolecular interaction energy into those that are insensitive (ΛΛ contributions) and sensitive (ΔΔ contributions) to molecular chirality.