A double many-body expansion of molecular potential energy functions

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Abstract
A suggestion is made to use a double many-body expansion of the molecular energy for determining the potential energy functions of small polyatomic systems. The approach, which consists in further partitioning the (n-body) energy terms in a many-body expansion of the total potential into Hartree-Fock [V (n) HF] and correlation [V (n) corr] energy contributions, generally uses the results from ab initio SCF calculations (or a function parameterized from available spectroscopic data) to represent V (n) HF while V (n) corr is obtained semiempirically, by interpolation from the dispersion energy coefficients of the various asymptotic channels of the potential surface. The method has been tested on the HeH2 van der Waals molecule and found to produce a potential which compares favourably with the most recent and reliable ones which have been published for this system. It is anticipated that this HeH2 Hartree-Fock-approximate correlation energy (HFACE) potential should be reliable both for studying vibration-rotation inelasticity in He-H2 and atomic hydrogen recombination in the presence of helium atoms.