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The orbital effect is investigated by the tight-binding method, using one-electron Hartree-Fock potentials. Two simple models are considered, the first consists of a BCC lattice with five orbitals and three electrons per atom, and the second consists of an FCT lattice with three d epsilon states and two electrons per atom. In both cases, when U 0 / Delta becomes a little larger than 1.0, a phase change occurs from metal to insulator, and around U 0 / Delta =1.0 the phase is semi-metal.
Shinya Wakoh (1977) studied this question.