A double many-body expansion of molecular potential energy functions

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.