Density functional (DFT) calculations were done using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional and the projector augmented wave (PAW) pseudopotentials as implemented in the Vienna ab-initio simulation package (VASP). The plane wave cutoff energy was set at 450 eV and a convergence criteria of 10 -6 eV and 10 -5 eV was employed for the electronic energy and the geometry relaxation, respectively. In addition, we included Grimme's semiempirical DFT-D3 corrections to account for the dispersion interactions. The Brillouin zone was sampled using a 1 × 1 × 1 Monkhorst-Pack mesh centered at the Γ point.