An expansion of the intermolecular energy in a complete set of symmetry-adapted functions; convergence of the series for methane–methane and adamantane–adamantane interactions

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Abstract
A general outline is given of the construction of a complete set of symmetry‐adapted functions, which can be used as a basis for an expansion of the intermolecular energy. It is shown that other terms may be present in this expansion besides those generated by a multipole expansion of the atom–atom potentials generally used to describe the intermolecular energy. Expressions are derived for the multipole expansion of Buckingham‐type potentials. Some partial sums are calculated for the purpose of studying the convergence of the infinite sum. The calculations are performed for several orientations of the interacting molecules. The examples studied are methane–methane and adamantane–adamantane interactions. It becomes clear that if only the first few terms in the expansion are used the results may be qualitatively wrong.