THE PHYSICS OF MOTT ELECTRON LOCALIZATION

Although rare earth metals and compounds are well known to have localized 4f electrons, the concept of "localized electrons" in a periodic system remains a notorious source of confusion. Similar issues arise for 3d electrons and have been studied most intensively for 3d transition metal oxides - the traditional Mott insulator materials. We review a conceptual picture of the Mott insulator state which clarifies the concept of localized electrons, and which has recently been implemented with much success. The key feature is strong orbitals as well as spin polarization, within the context of unrestricted Hartree-Fock theory using screened Coulomb and exchange parameters to represent the intra-atomic d-d interactions. The failure or success of local-density approximations is directly related to the presence or absence of orbital polarization. Some practical considerations and recent calculations are discussed.