THE ELECTROAFFINITY OF O-2 BY DFT AND COUPLED MCSCF PERTURBATION APPROACHES - A COMPUTATIONAL EXPERIMENT

The electroaffinity of the O-2 molecule is revisited using density fun ctional theory (DFT) and perturbation treatments built on a MCSCF wave function that includes most of the non-dynamic correlation effects (MC /P approach). Using a standard 6-31 + G basis set, DFT treatments bas ed on BLYP or B3LYP functionals provide electroaffinities of the order of +0.6 eV that compare favorably to experiment. Coupled MCSCF/pertur bation treatments using an Epstein-Nesbet partition of the molecular H amiltonian give a more accurate value of +0.492 eV in excellent agreem ent with the most recent experimental data (+0.431 eV) as well as with highest-level purely variational ab initio treatments which are far l ess tractable for larger systems. The analysis of the results in terms of differential correlation effects made it possible to identify the failure of the previous MCSCF-limited treatments as arising from the d ynamic correlation of the electron pair describing the sigma(o-o) bond .