Local exchange‐correlation approximations and first‐row molecular dissociation energies

Recent X α calculations of bond energies and other related properties of first‐row diatomic molecules show very encouraging agreement with experiment. In the worst cases, however, the X α dissociation energies overestimate the experimental values by almost 2 eV. Therefore, we have examined several refinements of the X α theory and their effects on molecular bond lengths, bond energies, and vibrational frequencies. Among them, gradient corrections to the X α exchange energy and also some variations of the local spin‐density correlation energy approximation are considered. We find that a local exchange‐correlation functional with gradient corrections gives dissociation energies in significantly better agreement with experiment than the X α approximation.