The calculated site-site transition frequencies for tetrahedral and octahedral sites as a function of their Ba/Sn coordination are shown in Fig. , normalised by the proportion of time each site type is occupied-this normalisation gives transition frequencies that are equivalent to average inverse residence times; higher transition frequencies correspond to fluoride ions leaving a particular site more quickly. The calculated site-site transition frequencies for both tetrahedral and octahedral sites generally increase with increasing degree of Sn-coordination, with this effect particularly strong for the tetrahedral sites. These data from AIMD simulation, therefore, are consistent with the model inferred from the variabletemperature NMR and fluoride-ion time-average density data (Figs. 6 & 8(b)): fluoride ions in "Sn-rich" sites are, in general, more mobile than fluoride ions in "Ba-rich" sites. We also note that the site-occupation-normalised transition frequencies are much higher for Ba-rich octahedral sites than for Ba-rich tetrahedral sites, which speaks to the relative stability of tetrahedral sites compared to octahedral sites in Ba-rich regions.