We argue that the widely held view that MnO, FeO, CoO, and NiO are Mott insulators is incorrect. Intra-atomic potential energies do not dominate interatomic kinetic energies, to the extent commonly believed; both are 1-2 eV in magnitude. A measure of the importance of interatomic effects is our finding that both the crystal structure and the magnetic structure of these materials are crucial to their insulating behavior.

• Received 24 February 1984

DOI:https://doi.org/10.1103/PhysRevLett.52.1830