The nonasil cage has 15 vertices. 12 of these vertices correspond to meeting points of three edges, and one (Si7) to a point where four edges meet. This results in a total of 14 structures with different fluoride positions (two separate ones with F bonded to Si7) for which DFT optimizations were performed. The results of these calculations are summarized in Figure , where all fluoride positions are shown within one nonasil cage, and colored according to their relative energy ΔErel with respect to the experimental position (the experimental position is defined as zero point of ΔErel, note that ΔErel values are always given per F atom). This position, labelled F@Si1_1, is the energetically most favorable site, with the second-best site being 5.3 kJ mol -1 higher in energy. All positions that are not associated with the 4MR are at least 18 kJ mol -1 higher in energy than the F@Si1_1 site.