Intermediate-Range Intermolecular Forces in H2+

A new type of molecular perturbation theory recently developed by Hirschfelder and Silbey is used to study intermolecular forces in the intermediate range where the wavefunctions overlap slightly and exchange must be taken into account. A method is outlined by which one can modify a nonexchange- or polarization-type first-order wavefunction to include the effect of exchange. Although the procedure seems to be general, for numerical convenience only the 1sσg and 2pσu states of H2+ are considered. The results are in good agreement with the exact energies for both states to approximately R=4 a.u., compared to about R=15 a.u. for a perturbation treatment without exchange.