Quantum Mechanics of the H2–H2 Interaction. V. The Importance of Intermolecular Charge-Transfer States

In a preceding paper a one-configuration wavefunction of strong orthogonal group functions was found to give a rather poor description of the orientation dependence of short-range intermolecular forces in H2–H2. As an alternative to the necessity of relaxing the strong orthogonality condition, it is suggested that the introduction of a small amount of the charge-transfer state H2−–H2+ describing one-electron transfer between the two molecules should improve the interaction energy and its change with the relative molecular orientation. Ab initio calculations in a region of intermolecular separations ranging from two to four times the internuclear equilibrium distance of an isolated H2 molecule show that less than 1% of such a charge-transfer state gives interaction energies which are in substantial agreement with those obtained with a complete configurational interaction.