Table reports the interaction energies ∆E int and adsorption enthalpies ∆H ads (for T = 298 K) obtained from the DFT calculations for fully hydrated AlPO 4 -H3 and for partially hydrated forms in which either one or two of the three non-equivalent water molecules were removed. There are eight water molecules of each kind in the unit cell. In the partially hydrated structures, all symmetry-equivalent images of the respective water molecule(s) were removed, retaining the Pbca symmetry. Optimised unit cell parameters and structure projections of these partially hydrated phases are given in the Supporting Information. For AlPO 4 -H3, the interaction energy of about -82 kJ mol -1 is more negative (i.e. the interaction is stronger) than energies obtained in analogous PBE-TS calculations for six AlPOs with larger pores and different topologies (among them AlPO-34 and AlPO-18), which amounted to approximately -65 kJ mol -1 . The difference can be explained with the ordered arrangement of water molecules in the narrow channels of AlPO 4 -H3, which maximises the number of hydrogen bonds per molecule. In contrast, a disordered distribution of the water molecules was assumed in the calculations for AlPOs with larger pores. It is also worth noting that the relative contribution of non-dispersive interactions is larger in AlPO 4 -H3, amounting to about 70% of the total interaction energy, compared to ∼60 to 65% in other AlPOs, confirming the larger energetic contribution of hydrogen bonds (this contribution was obtained by calculating the difference of the "pure" PBE energies, without the TS dispersion energy, and dividing it by ∆E int ). While no enthalpy of water adsorption has been determined experimentally for AlPO 4 -H3, it should be noted that the calculated ∆H ads of -67 kJ mol -1 is considerably more negative than experimental values determined for systems like AlPO-18, which are typically in the range of -55 kJ mol -1 . This is in line with the higher temperature of dehydration: Whereas AlPO-18 can be dehydrated below 100 • C , dehydration of AlPO 4 -H3 occurs between 80 and 130 • C . Thus, both experiment and DFT calculations indicate that AlPO 4 -H3 interacts more strongly with water than AlPOs with larger pores. Nevertheless, it should be emphasised that one cannot expect quantitative accuracy from the calculations, which is why the PBE-TS enthalpy of adsorption should be considered as a semi-quantitative estimate. For the partially hydrated phases, we observe a rather large variation in the unit cell volumes. For example, the removal of H 2 O(1) leads to a structure in which both the Al1 and Al2 atoms are in trigonal-bipyramidal coordination, causing a significant reduction of the unit cell volume to 1473 Å 3 , whereas the other 2/3 hydrated models have volumes that are much closer to that of AlPO 4 -H3 (∼1800 Å 3 ). Nevertheless, the interaction energy -which effectively constitutes a combination of energetic contributions from host-guest interactions and from the overall deformations of the host structure -is very similar for all three 2/3 hydrated models. This is not the case for the 1/3 hydrated phases, where the ∆E int value of -45 kJ mol -1 obtained for the model containing only the H 2 O(1) molecule differs considerably from those of the other two models, which are in the range of -70 kJ mol -1 . A closer inspection of the structures shows that the water molecules are coordinated to framework Al atoms in all three structures, and that they participate in one or two short hydrogen bonds. The local environment of the water molecule thus provides no apparent explanation for the large difference in the interaction energy. However, the host structure is deformed to a varying extent: In the models containing only H 2 O(2)/H 2 O(pore), pronounced elliptical distortions of eight-rings/six-rings in the structure cause a reduction of the unit cell dimensions, with the unit cell volumes being 13/10% smaller than that of AlPO 4 -H3. Such a volume reduction does not happen in the model containing only H 2 O(1). While we observe an inverse correlation between unit cell volume and stability for the 1/3 hydrated phases, a correlation that is well-established for guest-free tetrahedral frameworks , this is not found for the 2/3 hydrated models. Altogether, these findings point to a complex interplay of the energetic contributions from hydrogen bonds, from local framework distortions (non-optimal bonding geometries/distorted coordination environments), and from global framework deformations (changes in unit cell volume).