All density functional theory (DFT) calculations were performed with Vienna Ab initio Simulation Package (VASP), using the recommended projector-augmented wave (PAW) potentials to simulate the core and valence electrons. The same converged plane wave energy cutoff (480 eV) and k-point meshes (5×5×5 for primitive, 5×5×1 for conventional) were used as in a previous study. The crystal structure (Figure ) was obtained from Materials Project was relaxed into the ground state using the generalised-gradient approximation (GGA) Perdew-Burke-Ernzerhof functional revised for solids (PBEsol) and the hybrid-DFT Heyd-Scuseria-Ernzerhof (HSE06) functional. The conventional unit cell was relaxed with no constraints on unit cell shape and volume, until the maximum force on any atom did not exceed 0.01 eV Å-1 , using a higher energy cutoff (620 eV)