Geometry optimisation and density of states calculations were performed using single primitive cubic (ICSD #25596) and rhombohedral (ICSD #280539) cells. To examine antiferromagnetic ordering, single point energy calculations were performed for a cubic 2 × 2 × 2 supercell. Mixed oxygen/fluorine calculations were performed by applying the virtual crystal approximation (VCA) so that anion sites are treated as a stoichiometric mixture of oxygen and fluorine potentials. External pressure values were specified as a tensor within the CASTEP geometry optimisations. Density of states processing and optical band gap calculations were performed using OptaDOS (v1.2). Analysis of octahedral distortion was performed by fitting of a minimum bounding ellipsoid using the PIEFACE (v1.1.0) software package.