Comparison between explicitly correlated and density functional theory calculations in mixed-valence model systems

Considering simple model problems, symmetric mixed-valence compounds are studied by both accurate ab initio configuration interaction (CI) and standard (B3LYP) density functional theory (DFT) calculations. The DFT estimates of the effective hopping integral t for mixed-valence compounds are generally smaller (by 20%) than the CI values, in contrast to those of the exchange coupling constant in magnetic systems which are usually overestimated. Qualitatively divergent behaviors of DFT versus CI results have been found in some cases. Possible origins of these discrepancies are discussed.