Density-functional theory (DFT) calculations are performed using the Vienna Ab-Initio Simulation Package (VASP) . The electron exchange and correlation effects are treated within the generalized gradient approximation (GGA) using the PBE functional . The electron-ion interaction is described by the projectoraugmented wave (PAW) scheme . The surfaces are modeled using periodic supercells that contain 12 atomic layers. A vacuum region of about 15 Å is used to decouple the material slab from its periodic image. The electric field in the vacuum region resulting from the two nonequivalent slab surfaces is quenched using a dipole correction. The wave functions are expanded into plane waves up to an energy cutoff of 500 eV. The surface Brillouin zone is sampled using a Γ centered 4 × 4 k-point mesh. All interfaces are structurally relaxed until the forces acting on the atoms are below 0.02 eV/ Å. Band structure calculations for defect-containing 2×2 surface unit cells were performed within 4×4 translational symmetry, in order to reduce spurious defect-defect interactions.