Periodic density-functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) code. The PBEsol functional was used to describe electron exchange and correlation, as we have previously found that this gives good results for lattice-dynamics calculations. The ion cores were modelled using projector augmented-wave (PAW) pseudopotentials with the valence configurations: Bi -6s 2 6p 3 ; O -2s 2 2p 4 ; S -3s 2 3p 4 ; Se -4s 2 4p 4 ; and Te -5s 2 5p 4 . The Kohn-Sham wavefunctions of the valence electrons were represented in a plane-wave basis with a kinetic-energy cutoff of 600 eV, and Brillouin-zone integrations were performed using Γ-centered Monkhorst-Pack kpoint meshes with 4 × 4 × 6 subdivisions. Both parameters were chosen to converge the absolute total energy and cell pressure to < 1 meV atom -1 and < 1 kbar (0.1 GPa) respectively. The size of the charge-density grids was determined automatically to avoid aliasing errors (PREC = Accurate in VASP), and non-spherical contributions to the gradient corrections inside the PAW spheres were accounted for (LASPH = .TRUE.).