The calculations in this study were performed using density-functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP) code. The ion cores were modelled with projector-augmented wave (PAW) pseudopotentials. An energy cutoff of 480 eV and Γ-centred k-point meshes with 5 × 5 × 5 and 5 × 5 × 1 subdivisions were chosen respectively to model the valence wavefunctions of the 11-atom primitive and 22-atom conventional unit cell. These parameters were chosen to converge the total energy to within 1 meV atom -1 . Further information on the convergence testing and input parameter selection is available in the electronic Supporting Information.