We obtain the time constant for the ring rotation from the simulations by extracting the dihedral angle between the phenyl groups of each of the linkers in the unit cell as a function of time. An example for one of the linkers in UiO-67 is shown in Figure . The plot shows that the dihedral angle mostly oscillates around the equilibrium value of ±31 , with occasional crossings through the planar configuration with a dihedral angle of 0 . We define a proper transition between the two minima as an event where the dihedral angle changes from a positive value above 10 to a negative value below 10 or vice versa. The graph shown in Figure thus shows three proper crossings during the simulation time. Counting the number of crossings for all ligands in the unit cell leads to the averaged time between transitions, ⌧ , which can be converted to the frequency, ⌫, both given in Table . The smaller oscillations around the equilibrium positions most clearly seen in the right part of Figure indicates the vibrational frequency associated with the torsion of the rings, ⌫ 0 , which can be considered an attempt frequency for the proper transition. From the simulations we estimate this frequency as an average from several molecules where periodic oscillations are clearly seen. Alternatively, we can identify the phonon modes which are involved in the ring torsion by calculating the change in dihedral angle between the phenyl rings when following the phonon eigenvectors. This procedure identifies six modes with frequencies of 2.36-2.39 THz as leading to a significant change in dihedral angle in UiO-67, while for IRMOF-10 we find nine modes in the range 2.28-2.56 THz (see animations in the ESI). These ring torsion frequencies are close to those estimated from the MD simulation (c.f. Table ). For both materials a number of other phonons with frequencies in the range 12-30 THz change the dihedral angle; however, these modes also involve other distortions of the molecules (selected examples are given in the ESI).