The four resulting fluoride positions, labelled F@Si1_1 to F@Si1_4, were considered in the DFT calculations. Despite the identical local environment, the energy difference between the lowestenergy site (F@Si1_2) and the least favorable position is far from insignificant, amounting to 12.3 kJ mol -1 (Figure ). It is noteworthy that pairs of sites where the Si atoms are at opposite ends of the same equatorial Si-O-Si linkage are relatively close in energy, a point that will be elaborated in the following subsection.