A novel interpretation of Hund's rule for two-electron molecular systems

Abstract

Two approximate equality formulations for the energy-component differences between a triplet and a singlet state of a two-electron molecular system with the same geometry are proposed: i.e. Δ V ee = Δϵ a, Δ E HF = Δϵ b. These equalities concerning the interpretation of Hund's multiplicity rule indicate a novel theoretical background. The validity of these relationships in different basis sets and under different nuclear separations was also investigated. No basis-set or bond-length dependence of these equality formulations were confirmed by the ab initio self-consistent field (SCF) results. The reasons for the above formulations and the possibility of extending them to many-electron systems are discussed.