Asymptotic exchange energies for H2

An analytical approximation of the asymptotic exchange energy of two interacting hydrogen atoms is obtained. This approximation depends only on functions of the internuclear distance R, which remain bounded as R→∞ and is derived using the Herring-Holstein surface-integral technique. It is found that, for large R, the exchange energy is O(R3exp(−2R)) in contrast to earlier approximations of O(R2.5exp(−2R)). Our result is similar to the classic Heitler-London expression without the unphysical term O(R3ln(R)exp(−2R)).