Initial guesses for the crystal structures were obtained from the Materials Project, before relaxing the geometry using the HSE06 screened hybrid DFT functional. The plane-wave energy cutoff and Γ-centered k-point mesh were sequentially increased using vaspup2.0 77 until total energies from static calculations were converged to 0.1 meV/atom -giving values of 300 eV and 3 × 3 × 3 (for the 9-atom primitive unit cell, equivalent to a k-point density of 0.33 Å-1 in reciprocal space) respectively. During structural optimization, a convergence criterion of 0.01 eV/ Å was imposed on the forces on each atom, and the plane-wave energy cutoff was increased to 500 eV, as well as re-relaxing the final geometries, to avoid Pulay stress effects. As discussed at the beginning of the results section, the effect of dispersion corrections (Grimme's D3) on the structural relaxation was tested and shown to be important; and so the HSE06+D3 (using the recommended PBE0+BJ parameterizations) relaxed unit cells were used for all further calculations in this work.