A study of molecular pair potentials; A semiempirical approach

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Abstract
An expression is developed for the short range molecular pair potentials within the framework of the Heitler–London approach for molecules in a closed shell state. Assumptions common with the CNDO/2 semiempirical method are employed in evaluating the molecular integrals. The final expression depends only on quantities related to the atoms making up each molecule. There are four such quantities; (1) the atom charge density Pm; (2) the atom core charge Zcm; (3) the distance between atoms rmn; and (4) the value of two electron repulsion integrals between atoms γmn. In terms of these quantities the final expression has the simple form E(1)ab= (−1) Σm(a)Σn(b) [PmPnγmn(Zcm/Pm+Zcn/Pn −1)−ZcmZcnr−1mn]. Although several approximations are introduced to obtain this expression these are related to the evaluation of integrals, and no adjustable parameters are required for the evluation of E(1)ab. This potential is in good agreement with the results of ab initio calculations for the system He/H2, Ne/H2, and HF/HF. A comparison is also made with the results of the Gordon–Kim method for the system HF/HF. Both E(1)ab and the Gordon–Kim method are found to give comparable results, but the method presented in this work is found to be computationally ∼3000 times faster per data point.