The structures were relaxed using both the Perdew-Burke-Ernzerhof generalised gradient approximation (GGA) functional revised for solids (PBEsol) and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional , with no constraints on the unit-cell shape or size, until the maximum force on any atom did not exceed 0.0005 eV A -1 . During these calculations a 30% higher cutoff (620 eV) was employed to account for Pullay stresses. The electronic band structure and density of states (DOS) were calculated with HSE06 and analysed using the sumo package, which was also used to evaluate the effective charge-carrier masses using parabolic fitting of the band edges.