Using static and dynamics DFT methods we show that the Mg2+/F-/phosphate/water cluster at the center of the fluoride riboswitch is stable by its own and, once assembled, does not rely on any additional factor from the overall RNA fold. Further, we predict that the pK(a) of the water molecule bridging two Mg cations is around 8.4. We also demonstrate that the halide selectivity of the fluoride riboswitch is determined by the stronger MgF bond, which is capable of keeping together the cluster. Replacing F with Cl results in a cluster that is unstable under dynamic conditions. Similar conclusions on the structure and energetics of the cluster in the binding pocket of fluoride-inhibited pyrophosphatase suggest that the peculiarity of fluoride is in its ability to establish much stronger metal-halide bonds.