Repulsive intermolecular potential between two H2 molecules

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Abstract
Ab initio calculations of the intermolecular potential Φ between two H2 molecules have been carried out for intermolecular separations corresponding to the repulsive region of Φ. In most of the calculations a contracted Gaussian basis set consisting of two s and one p functions per H was used to construct a self‐consistent field (SCF) configuration and SCF plus all singly and doubly excited configurations at fixed H–H bond length. To ascertain the accuracy of the computed results, three types of more elaborate calculations were carried out. These are based on a larger Gaussian basis set (five s and two p functions per H), full configuration interaction, and variation of the H–H bond length. We found that Φ depends weakly on these factors at r≲4.5 bohr. Analytic expressions are presented to represent Φ in terms of intermolecular distance and three angles defining orientations of two H2 molecules. The minimum‐energy orientation of two H2 as they approach each other is predicted to have a simple shape, two H2 forming a parallelogram. The results obtained in this work are compared with those for other Φ’s obtained from theoretical and experimental considerations. Since Φ’s based on the condensed‐state data are less repulsive than the ab initio Φ, we conclude that the many‐body effects in condensed states are significant over a nearly entire range of repulsion.