In addition to the three experimentally observed sites, there are seven other corners of the [4 •5 4 ] cage to which a fluoride anion could be bonded. The DFT results for these 10 fluoride locations are summarized in Figure . Remarkably, the experimentally observed position with the highest occupancy (F@Si12) has the lowest energy, with the other two experimental sites lying within about 2.5 kJ mol -1 . With the exception of the F@Si6 site (ΔErel = 3.9 kJ mol -1 ), all other configurations are at least 9 kJ mol -1 higher in energy. As in the cases of NON and IFR, there is a clear preference for positions associated with 4MRs, but not all of these positions are energetically favorable.