Deviations from Pairwise Additivity in Intermolecular Potentials

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Abstract
The various contributions to the interaction energy as defined in the theory of intermolecular forces in the region of small orbital overlap [J. N. Murrell, M. Randic, and D. R. Williams, Proc. Roy. Soc. (London) A284, 566 (1965)] are generalized to many‐body systems. Each contribution is examined in a qualitative manner for deviations from pairwise additivity. Numerical calculations of the exchange energy for three helium atoms in triangular and linear arrays are made utilizing the available expansions of Slater‐type orbitals in terms of a Gaussian set. The nonadditive terms for these energies are then calculated over a range of interacting distances. It is concluded that for weakly interacting nonpolar molecules nonpairwise additive terms in the intermolecular potential are likely to be small.

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