The inequality formulation of Hund's rule and a reinterpretation of singlet-triplet energy differences, generalized for molecules at equilibrium geometry
An inequality formulation in the open-shell Hartree-Fock approximation for Hund's multiplicity rule for atoms has been generalized for the energy differences between a singlet and a triplet state of a molecule with different geometries for the two states. A numerical example is given for the lowest 1Πu and 3Πu states of H2. As in neutral atoms the energy difference is dominated by the difference in electron nuclear attraction and the interelectronic repulsion is higher for the triplet state than for the singlet state. The π orbital is more contracted in the triplet state than in the singlet state; the σ orbital gives a minor expansion in going from the 1Πu state to the 3Πu state.