A study of molecular pair potentials, a semi-empirical approach

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 semi-empirical method are employed in evaluating the molecular integrals. The final expression depends only on quantities related to the atoms making up each molecule. 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 evaluation of the potential. 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. The method presented in this work is found to be computationally approx. 3,000 times faster per data point than the Gordon-Kim method with comparable results.