Generalized-gradient-approximation description of band splittings in transition-metal oxides and fluorides

The local-spin-density approximation yields a metallic ground state for the antiferromagnetic insulators CoO, FeO, FeF₂, and CoF₂. By using the generalized gradient approximation (GGA) we find the difluorides to be insulators, while in CoO the GGA splits the bands near EF and opens a direct gap, but a small indirect band overlap remains. For FeO a significant improvement is found but not quite enough to make it an insulator. The magnetic moments and gaps increase. The GGA improves angular (and also in-out) correlations and thus affects the energy bands, a feature in addition to the total-energy improvement.