Singlet-triplet energy component differences in homonuclear diatomics: A multi-reference configuration interaction study of Hund's rule

Abstract

Multireference configuration interaction calculations for the low-lying electronic states of C 2 (a 3Π u and A 1Π u) and O 2 (X 3Σ g −, a 1Δ g) over a range of internuclear separations reveal that the singlet-triplet electron-electron repulsion energy differences tend to have the same sign as the singlet-triplet kinetic energy differences, while the corresponding electron-nuclear attraction energy differences tend to have the opposite sign at a given geometry. Adiabatic energy component differences for C 2 and O 2 show that the electron-electron repulsion in the triplet state is greater than in the singlet state, while there is more electron-nuclear attraction in the triplet than in the singlet state. The latter observations are consistent with the results for atomic states arising from the same configuration, but are contrary to the traditional explanation of Hund's rule.