In the STF, F@Si3 structure, the dynamic events are primarily associated with a change of the x coordinate (Figure ). This corresponds to a jump between the F@Si3 site and the other apical site, F@Si4, with the fluoride anion following an arc-like trajectory as observed above for IFR. The mobility between these two sites is not surprising in the view of the DFT optimization results, as they are spatially and energetically close together. In the STF, F@Si10 case, the dynamic events also coincide with displacements along the a direction, corresponding to a mobility between the F@Si10 and F@Si1 sites. According to the DFT optimizations, the energy difference between these two sites amounts to about 8 kJ mol -1 . In the light of this sizeable energy difference, it seems plausible to expect that dynamic events occur only at elevated temperatures, where the thermal energy is sufficient to overcome the energetic barrier. In the first case (Figure ), the fluoride anion moves diagonally across the 4MR, from the F@Si12 site (the initial position) to the F@Si7 site, another one of the three experimentally observed fluoride locations. Figure actually shows two consecutive events, with fluoride again moving from F@Si12 to F@Si7, but with a transient location at F@Si4. The bonding of fluoride to the Si4 atom is short-lived, lasting only for about 1 ps. The apparent instability of this fluoride location can be explained straightforwardly with its much higher energy (ΔErel = 18.4 kJ mol -1 ). In the scenario depicted in Figure , the fluoride anion is located at the F@Si7 site at the beginning, so it must have moved to this position during the equilibration phase.