An irreducible tensor method of deriving the long-range anisotropic interactions between molecules of arbitrary symmetry

Abstract

The long-range, anisotropic interaction energy between two molecules of arbitrary symmetry is derived to second order in perturbation theory by using irreducible tensor methods. The resultant formulas can be used to express the interaction energy to any desired power of R, the separation of the two molecules. Group-theoretical arguments are used to reduce the number of independent parameters in the theory and to determine which parameters are nonzero in particular molecular point groups. The theory is illustrated by being applied to the case of two linear molecules, and the interaction energy is given explicitly for linear molecules up to R−5 for the electrostatic (first-order) interactions and to R−7 for the induction and dispersion (second-order) interactions. Certain discrepancies in the published literature concerning the induction and dispersion interactions of two linear molecules are clarified.