Correlation between Exchanging Electrons

Abstract

Attempts to introduce overlap and exchange into a purely general study of interatomic or intermolecular long‐range forces lead to complicated, physically opaque expressions for the interaction energy. A study is made of a most simple system, two hydrogen atoms at large distances, taking overlap and exchange rigorously into account. This system is especially interesting due to the failure of the Heitler—London wavefunction to describe adequately the lowest‐triplet (³Σ_u⁺)—lowest‐singlet (¹Σ_g⁺) separation at very large distances. A formal study separates the interaction energy into a ``generalized dispersion'' and ``generalized exchange—correlation'' energy. A many‐configuration calculation performed at an internuclear distance of 8 a.u. shows that, whereas certain configurations yield the well‐known dispersion energy, other configurations, particularly those of the excited charge‐transfer type, correct for the erroneous triplet—singlet separation. This distinction reflects the introduction of two distinct types of correlation effects which are investigated in terms of the two‐electron distribution function.