For each composition x, the cubic ReO3 lattice parameter was optimised enforcing FM ordering. In each case, the minority spin band gap and overall spin density have been extracted and are summarised in Fig. . NbO2F is correctly predicted to be a semiconductor with a band gap of 3.4 eV, in close agreement to the experimental value of 3.1 eV. The PDOS for NbO2F (Fig. ) shows the valence band is dominated by the O 2p states whilst Nb 3d states dominate the conduction band. Comparing this result with the PDOS for NbO1.8F1.2 (Fig. ), the effect of increasing the fluorine content is to reduce Nb(V) towards Nb(IV), donating electrons into the conduction band. This leads to a transition from semiconductor to metal for small x.