To obtain another perspective on the degree of fluoride ion disorder, we project the instantaneous fluorideion positions from our AIMD simulation trajectory onto discrete tetrahedral or octahedral sites, defined by the set of Wyckoff 4a sites that define their vertices. This siteprojection gives a remarkable 1/3 of fluoride ions occupying octahedral "interstitial" sites rather than conventional tetrahedral sites-i.e., individual octahedral sites are, on average, equally likely to be occupied by fluoride ions than individual tetrahedral sites. This degree of fluoride-ion site-disorder is even greater than the "massive disorder" found in RbBiF 4 , where 1/4 of fluoride ions occupy nominally octahedral positions . Furthermore, this disorder is not simply a large number of thermally generated anion "Frenkel pairs": quenching from our AIMD simulation produces a 0 K structure with this same proportion of fluoride ions occupying octahedral sites that is 16.7 meV/atom lower in energy than the corresponding optimised structure with all fluoride ions occupying tetrahedral positions. This extreme fluorideion disorder is therefore intrinsic to c-BaSnF 4 .