The root mean square displacements (RMSDs) obtained from the AIMD simulations are shown in Table . For the framework atoms, we find RMSDs in the range of 0.14 Å for aluminium and phosphorus atoms, and RMSDs of 0.18 to 0.23 Å for the bridging oxygen atoms. Unsurprisingly, the variations among the non-equivalent atoms of each group are not significantly larger than the typical standard deviations, in line with the expected rigidity of the framework. When looking at the three different water molecules, a much larger variation of the RMSDs is found: For the H 2 O(2) molecule, the RMSD of the O10 oxygen atom of 0.19 Å is similar to those of framework O atoms, indicating that this strongly bonded water molecule exhibits no significant mobility. The RMSDs of the two attached hydrogen atoms, H3 and H4, are only slightly larger, because the participation in strong and short hydrogen bonds limits the extent of displacements from their equilibrium positions. For the H 2 O(1) molecule, the RMSD of the O9 oxygen atom is only slightly larger than that of the O10 atom and of the framework oxygens. On the other hand, the RMSDs of the H1 and H2 atoms are much larger, amounting to 0.36 and 0.51 Å, respectively. While it seems plausible that these hydrogen atoms, which participate in longer hydrogen bonds than those of the H 2 O(2) molecule, undergo more pronounced oscillations, it has to be noted that the H2• • •O11 hydrogen bond in the PBE-TS equilibrium structure is much shorter than the H1• • •O11 bond. Therefore, one should expect a smaller RMSD for H2 compared to H1, contrary to the results of the AIMD calculations. We will revisit this point below, when discussing the radial distribution functions. Finally, the RMSDs for all three constituent atoms of the H 2 O(pore) molecule are in the range of ∼0.55 Å, much larger than for all other atoms except H2. Apparently, this water molecule is subject to pronounced oscillations, which is straightforwardly explained with the absence of an Al-O water bond, leading to a significant freedom of movement along the channel. A visualisation of the positions visited by a single O11 atom over the course of the whole AIMD trajectory shows the large displacement of the H 2 O(pore) molecule (Figure ). The individual x, y, and z coordinates over the trajectory vary by 0.8/1.0/1.2 Å, corroborating that the most pronounced displacement occurs along the channel axis (c-axis). Given the large mobility of the H 2 O(pore) molecule found in the AIMD simulations, it is not surprising that the static equilibrium position of O11 obtained from the PBE-TS optimisations deviates from the experimentally observed position, which corresponds to the time-averaged position at room temperature. Isostropic displacement parameters U iso can be obtained from electronic structure calculations via two different routes: On the one hand, they can be calculated from an AIMD trajectory, where they are directly related to the RMSDs. On the other hand, anisotropic displacement parameters can be routinely computed in the context of a lattice dynamical calculation, and the conversion into an equivalent isostropic value is straightforward. Table lists both types of DFT-derived U iso values, dubbed as U iso (AIMD) and U iso (phonon), respectively, as well as experimental values . If we compare the displacement parameters for all atoms except those belonging to the water molecules (i.e. including Al, P, and O1 to O8 atoms), a purely qualitative analysis indicates reasonably good agreement with experiment for both types of computed U iso values. A more quantitative assessment can be made by plotting the DFT-derived values as a function of their experimental counterparts. These two plots are shown in the Supporting Information (Figures and). Here, we find excellent agreement between the U iso (AIMD) values and experiment, with a correlation coefficient R 2 = 0.93 and a slope of the trendline of 0.98 (perfect agreement would result in a slope of 1). The predicted values obtained from the phonon calculation are in somewhat less good agreement, with a correlation coefficient of R 2 = 0.82 and a slope of 1.13, i.e. the calculations tend to overestimate U iso . However, given the likely uncertainties of both experimental and calculated values, this agreement can still be considered satisfactory. With regard to the water molecules, we observe rather good agreement between both kinds of calculated U iso values and experiment for the oxygen atoms O9 and O10, which belong to the Al-coordinated water molecules. In contrast, the DFT-derived values for the hydrogen atoms are always much smaller than experimental values, which reach up to 0.20 Å. However, we have to note that the experimental values are likely to be inaccurate due to the difficulties in determining the hydrogen positions accurately (note especially that the O-H distances and H-O-H angles were fixed in the structure refinement, and that deviations in the real structure from these ideal values would translate into an increase of the displacement parameters). As such, we consider the experimental displacement parameters unreliable, and focus on a comparison of the different DFT-derived values. The U iso values obtained for the hydrogen atoms of the H 2 O(2) molecule differ appreciably among the two methods, but both methods agree on their relatively modest magnitude (U iso < 0.03 Å). For the H 2 O(1) molecule, the phonon calculation delivers increased U iso values, which are similar for both hydrogen atoms. As discussed above, a different picture arises when looking at the AIMD results, where the displacement parameter of the H2 atom is more than twice as large than that of H1. For the H 2 O(pore) molecule, the two computational methods give very different displacement parameters for all three atoms, with the U iso (AIMD) values being a factor of 2 to 3 larger than those from the phonon calculations. The phonon calculations only consider harmonic oscillations around the equilibrium position, however, the significant displacements of the H 2 O(pore) molecule predicted by the AIMD calculations indicate that the harmonic approximation is no longer valid. As these effects are captured only by the molecular dynamics approach, we may consider the displacement parameters obtained with this method to be more reliable. Interestingly, the experimental U iso value of the O11 atom falls between the two computed values. However, the fact that the hydrogen positions of the H 2 O(pore) molecule could not be determined is likely to cause a rather large inaccuracy of this displacement parameter. Table . Root mean square displacements and isotropic discplacement parameters U iso obtained from AIMD calculations as well as U iso obtained from phonon calculations. Experimental values are given for comparison . The radial distribution functions (RDFs) of the H• • •O hydrogen bonds obtained from the AIMD calculations are visualised in Figure to S7 of the Supporting Information. For the H 2 O(2) molecule, both the H3• • •O5 and the H4• • •O7 RDF exhibit one sharp first maximum centered at about 1.73 Å and 1.60 Å, respectively, in excellent agreement with the equilibrium H• • •O distances obtained in the PBE-TS structure optimisation (Table ). Looking at the H 2 O(1) molecule, the H1• • •O11 RDF has a somewhat broader maximum at a distance of ∼1.73 Å. This is significantly shorter than the PBE-TS equilibrium distance of 2.06 Å, a difference that can be attributed to the mobility of the H 2 O(pore) molecule, which is not accounted in the static calculation. For the H2 atom, there is one maximum in the H2• • •O11 centered at ∼1.84 Å, which agrees well with the PBE-TS distance. Additionally, a pronounced tail is observed towards longer distances, and a calculation of the RDF between H2 and different framework oxygen atoms reveals that there are also short contacts to the framework oxygen atom O6 occurring over the course of the AIMD simulation. The first maximum in the H2• • •O6 RDF is close to ∼2 Å, well below the PBE-TS equilibrium distance of 2.88 Å. This indicates that the hydrogen bonding situation of the H2 atom is dynamic, rather than static, with H2• • •O11 and H2• • •O6 hydrogen bonds forming and breaking over time (the environment of the H2 atom including both O11 and O6 is shown in Figure ). The dynamic H-bonding of H2 also explains why its U iso (AIMD) value is so much larger than U iso (phonon). For the H 2 O(pore) molecule, the RDFs for the H6• • •O4 hydrogen bond and the bifurcated hydrogen bond (H5• • •O1, H5• • •O8) all exhibit a single maximum centered at the respective equilibrium distances. These maxima are relatively broad due to the high mobility of the H 2 O(pore) molecule. The RDFs of intramolecular O-H bonds and of Al-O bonds are also included in the Supporting Information (Figures and). In all cases, the position of the first maximum in the RDF hardly deviates from the equilibrium distance in the PBE-TS optimised structure. For both groups of bonds, the maxima become broader with increasing equilibrium bond length. This effect is particularly pronounced for the Al-O bonds, where bonds to water molecules are somewhat more flexible than bonds to framework oxygen atoms.