For primitive cell calculations, the Brillouin zone was sampled by odd number k-point meshes with spacing less than 0.25 Å -1 , using the Monkhorst-Pack scheme . The electron wave functions were expanded in plane waves up to a kinetic energy cutoff of 650 eV, and the self-consistent field convergence energy was set to 10 -4 eV. Full structural relaxations of the cell volume and atomic positions were performed until the residual forces on the ions converged to below 0.05 eV/Å -1 . Subsequently, the cell volume and atomic positions were fixed, and the total energies were calculated using the tetrahedron method with Blöchl corrections .