The electronic structure was modelled using a plane-wave basis set with a 500 eV kinetic-energy cutoff and Γ-centered Monkhorst-Pack k-point meshes with 4 × 4 × 4 subdivisions. These values were determined from explicit testing to converge the total energy to < 0.1 meV atom -1 and the external pressure to < 1 kbar (0.1 GPa). Electron exchange and correlation were modelled using the PBEsol generalised-gradient approximation (GGA) functional, with a Hubbard 𝑈 correction of 𝑈 eff = 5 eV applied to the Co 3d states using the rotationally-invariant Dudarev model. Some additional tests were performed including the semi-empirical DFT-D3 dispersion correction. Projector augmented-wave (PAW) pseudopotentials were used to model the ion cores, with the following valence electron configurations: Co -3p 6 4s 2 3d 7 , Sb -5s 2 5p 3 , He -1s 2 , Ne -2s 2 2p 6 Ar -3s 2 3p 6 , Kr -4s 2 4p 6 , and Xe -5s 2 5p 6 . Tolerances of 10 -8 eV and 10 -2 eV Å -1 were applied to the electronic wavefunctions and ionic forces during electronic structure solution and geometry optimisation, respectively. The PAW projection was performed in reciprocal space, non-spherical contributions to the gradient correction were included inside the PAW spheres, and the precision of the charge-density grids was automatically set to avoid aliasing errors.