The comparison of density functional theory cluster and slab approaches is presented for modeling the formation of electrically pinned and unpinned metal oxide-III/V semiconductor interfaces. Thermodynamic stability, interfacial electrical properties, interfacial charge trap formation and bonding structures are examined critically in the case of gallium oxide formation on the GaAs(001)-,beta2(2 X 4) surface via direct oxidation of the surface with thermal O-2(g) and by vapor deposition of Ga2O(g). It is seen in both cluster and slab models that the direct oxidation with thermal O-2 will lead to an electrically pinned surface, while vapor deposition of Ga2O Will electrically passivate the surface, effectively unpinning the interface. Fermi-level pinning and unpinning is observed in the local density of states (DOS) in the band-gap region, in the charge distribution per surface atom, and in the geometric structures. It is seen that the DOS can be accurately predicted using either cluster or slab DOS. When cluster DOS is calculated, band-gap states appear delocalized due to poor global convergence caused by the finite cluster size effect. The thermal smearing factor for the density of states needs to be decreased from the typical value of 0.2-0.1 eV to compensate for poor convergence to reproduce accurate DOS. While cluster and plane-wave slab models predict the experimentally observed phenomenon, the slab models more accurately predict the reaction thermodynamics. We have compared both linear combination of atomic orbital (LCAO) clusters to plane-wave slab models and plane-wave clusters to plane-wave slab models to investigate the most critical parameters in attaining accurate results. It is seen that both the LCAO and plane-wave cluster models are poorly converged with respect to total energy due to the finite cluster size effect, causing over 1 eV error in the total energy. (C) 2003 American Vacuum Society.