The inequality formulation of Hund's rule and a reinterpretation of singlettriplet energy differences, generalized for molecules at equilibrium geometry
Abstract
An inequality formulation in the openshell HartreeFock 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 ^{1}Π_{u} and ^{3}Π_{u} 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 ^{1}Π_{u} state to the ^{3}Π_{u} state.
 Publication:

Journal of Chemical Physics
 Pub Date:
 May 1978
 DOI:
 10.1063/1.436292
 Bibcode:
 1978JChPh..68.4248C
 Keywords:

 31.15.+q;
 35.20.Bm