Band theory based on the spin-polarized self-consistent-charge extended Hückel tight-binding (SP-SCC-XHTB) method, which includes all the relativistic effects and the spin-polarized self-consistent-field (SP-SCF) atomic-structure calculation based on the Hartree-Fock-Slater method, has been applied to the electronic-structure calculations of tetragonal rutile-type MO S, such as nonmagnetic (n) TiO , VO , NbO , TaO , CrO , MnO , ferromagnetic (f) CrO and antiferromagnetic (af) MnO . It is shown that the calculations are consistent with the experimental observations for all systems except the case of MnO . It is demonstrated that the semiconductive nature of MnO can be explained by adding an assumption that three electrons in the Mn t orbital are localized in the crystal into the SP-SCC-XHTB band-structure calculation. © 1998 Elsevier Science S.A. All rights reserved.