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 H 2. 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. © 1978 American Institute of Physics.68942484251