Insulating gap in FeO: Correlations and covalency

We report calculations of the electronic structure of FeO in the local-density approximation (LDA) and LDA+U approximation with and without rhombohedral distortion. In both cases LDA renders an antiferromagnetic metal, and LDA+U opens a Hubbard gap. However, the character of the Sap is qualitatively different in the two structures, and the difference can be traced down to the underlying LDA band structure. An analysis of the calculations gives insight into the origin of the insulating gap in 3d monoxides and into the role of the k dependency of U, missing in the contemporary LDA+U method.