All first-principles calculations were performed using a DFT+U approach implemented in CASTEP (v18.1). The generalised gradient approximation (GGA) Perdew-Burke-Ernzehof (PBE) exchange-correlation functional was used for all calculations. A Hubbard U parameter of 3.5 eV was chosen to match, as closely as possible, the previously predicted NbF3 spin down band gap (Fig. ). The electronic ground state was found by the density-mixing algorithm with a mixing amplitude of 0.6 and 1.5 for the charge and spin density respectively. The electron wave functions were expanded as plane waves up to a cut-off energy of 850 eV.