In conclusion we have investigated the origin of the large thermal displacements of the organic BPDC linker in the MOFs UiO-67 and IRMOF-10. We identify the equilibrium localminumum structure of the two materials as having a dihedral angle of ±31 . We calculate the underlying double-well potential and show that switching between the two wells happens on the picosecond time scale at room temperature. The calculated energy barrier for this process is similar in the two materials, suggesting that switching is largely independent of the nature and connectivity of the metal clusters, and thus would be found in other MOFs containing this particular ligand, or ligands of the same family. This should be considered in the crystallographic refinement procedures and computational structure optimisation as the properties of the static average structure are different from the true dynamic system.