The adsorption of water in aluminophosphate zeotypes (AlPOs) is of particular interest due to their potential application in adsorption-based heat transformations using water as a working fluid . Moreover, it is also interesting on a more fundamental level, as previous work has shown that two types of adsorbed water molecules can be distinguished in hydrated AlPOs, namely water molecules that are bonded to framework Al atoms, leading to a change in the coordination environment of aluminium to fivefold (trigonal-bipyramidal) or sixfold (octahedral) coordination, and (2) water molecules that reside in the pores, which interact with oxygen atoms of the pore wall or of other water molecules through hydrogen bonds. Experimental evidence for the existence of "Al-coordinated" and "pore" water molecules has been obtained for several AlPOs with varying pore sizes using a portfolio of experimental methods, including NMR spectroscopy , single-crystal X-ray diffraction (sc-XRD) , powder X-ray diffraction (PXRD) , and vibrational (Raman) spectroscopy . For water molecules coordinated to framework Al atoms, typical Al-O water bond lengths are in the range of 1.95 to 2.05 Å , which is close to the sum of ionic radii of 1.935 Å . The formation of relatively short Al-O water bonds could indicate that the interaction with these water molecules is stronger than for "pore" water molecules that are bonded only through hydrogen bonds. If this was the case, it would imply that partially hydrated AlPOs that contain only Al-coordinated water molecules, but no additional adsorbed water molecules in the pores, should exist as an intermediate phase between the dehydrated and fully hydrated forms. However, joint experimental and computational studies of hydrated AlPOs with medium-sized to large pores like AlPO-34 (CHA topology) , AlPO-18 (AEI topology) , and an LTA-type AlPO have provided no evidence for the existence of such partially hydrated phases. Density functional theory (DFT) calculations for AlPO-34 delivered a binding energy of -14 kJ mol -1 for one water molecule per chabazite cage (one five-coordinated Al atom) . For four molecules per cage (two six-coordinated Al atoms), a binding energy of -49 kJ mol -1 per H 2 O was calculated, which is still significantly less than the value of -67 kJ mol -1 per molecule obtained for a fully hydrated phase with ten H 2 O molecules per cage. Such an increase of the interaction energy with increasing number of adsorbed molecules is contrary to the trend observed in most adsorption processes, where the heat of adsorption decreases with coverage because the energetically most favourable sites are occupied first. It points to a dominant role of water-water interactions. On the basis of these calculations, it was concluded that a complete filling of the pore occurs once the chemical potential of the water vapour is high enough to allow the formation of six-coordinated Al atoms, and that there is no stable intermediate phase that contains only Al-coordinated water molecules. In other words, hydrogen bonds to water molecules in the pores are necessary to stabilise Al-coordinated water molecules in AlPO-34 and, presumably, also in other AlPOs with pores of a similar size. The situation is different in non-porous hydrated aluminophosphates like variscite and metavariscite, where only Al-coordinated water molecules are present, which form hydrogen bonds to framework oxygen atoms . AlPO 4 -H3 is a hydrated aluminophosphate zeotype with composition AlPO 4 •1.5H 2 O that was first synthesised by d'Yvoire . Its crystal structure was determined from single-crystal data by Pluth and Smith . The framework, which was assigned the APC framework type code by the International Zeolite Association , consists of PO 4 tetrahedra (atoms P1, P2), AlO 4 tetrahedra (Al1), and AlO 4 (H 2 O) 2 octahedra (Al2), with each framework oxygen atom being bonded to one Al and one P atom (i.e. all linkages are Al-O-P linkages). The APC framework can be described as a stacking of 4 • 8 2 layers along the c-axis, leading to straight channels bordered by eight-membered rings that run along c. The channels are interconnected by eight-ring windows along a, forming a two-dimensional channel system. The whole structure can be built up from a single building unit (natural tile) with the face symbol [4 5 • 6 2 • 8 3 ] . There are two different types of water molecules in the structure of AlPO 4 -H3: In addition to the two Al-coordinated water molecules that are part of the octahedra (called H 2 O(1) and H 2 O(2) throughout this work), a third water molecule H 2 O(pore) is located in the channels that run along c (see Figure , a more detailed description of the environment of the different water molecules is given below). Dehydration of AlPO 4 -H3 leads to AlPO 4 -C, which has the same topology . The dehydration occurs between 80 and 130 • C, and thermogravimetric measurements show that all water molecules are removed during a single mass loss step . The dehydration is reversible at low temperatures, but heating to about 250 • C leads to a topotactic transformation to AlPO 4 -D (APD framework type) . AlPO 4 -D can in turn be rehydrated to form AlPO 4 -H6. While both the APC and the APD framework can be described as a stacking of 4 • 8 2 layers, the chains connecting these layers differ (double-crankshaft chains in APC, nasarsukite-type chains in APD) . This article presents an in-depth computational study of the hydrated aluminophosphate AlPO 4 -H3 by means of dispersion-corrected DFT. The aim of this study is threefold: First, the interaction strength with water in AlPO 4 -H3 and partially hydrated models of APC-type AlPOs is evaluated in order to elucidate the potential (in)stability of partially hydrated phases. Second, the vibrational spectra of AlPO 4 -H3 and dehydrated AlPO 4 -C are predicted using density functional perturbation theory. By identifying modes that are present in the hydrated form, but not in the dehydrated structure, and by analysing the displacement patterns associated with these modes, it is attempted to propose "fingerprint" modes that are indicative for water molecules in different environments. Finally, DFT-based ab-initio molecular dynamics (AIMD) calculations are performed to analyse the dynamic behaviour of AlPO 4 -H3, with a particular focus on the dynamics of the water molecules. the lattice parameters as well as selected average bond lengths obtained in DFT calculations using the PBE-TS functional to experimental values. For the case of AlPO 4 -H3, the most significant deviation in the lattice parameters is an overestimation of the length of a by 0.15 Å (0.75%), whereas b and c agree to within 0.03 Å (0.3%). The lengths of the bonds between framework oxygen atoms and tetrahedrally coordinated Al and P atoms (d(Al1-O fw ), d(P-O fw )) and octahedrally coordinated Al atoms (d(Al2-O fw )) are moderately overestimated by 0.015 to 0.025 Å. The same applies for the bonds from octahedrally coordinated aluminium to the water molecules (d(Al2-O9), d(Al2-O10)). With lengths of 1.981 Å and 1.965 Å, respectively, these bonds are roughly 0.1 Å longer than the average Al2-O fw distance. The general tendency to deliver too long bonds is in line with previous work, where an overestimation of T-O bond distances (T = tetrahedrally coordinated atoms) in all-silica zeolites and aluminophosphates was observed for PBE-based approaches, independent of the presence/absence of a dispersion correction . While this is partly an inherent feature of this functional, it was also shown that the agreement between DFT and experiment can be improved if the experimental bond distances are corrected for the correlated motion of TO 4 tetrahedra . Indeed, correcting the experimental bond distances using the "simple rigid bond" (SRB) model of Downs et al. brings experimental and computed values closer together (Table ), but the tendency to overestimate the T-O bond lengths remains prominent. For the case of AlPO 4 -C, the agreement in lattice parameters is markedly less good than for AlPO 4 -H3, as the length of a is overestimated by almost 0.6 Å (∼3%), whereas b is underestimated by 0.4 Å (∼4%). For the T-O bond lengths, there is a trend to overestimate these distances by roughly 0.02 Å, as for AlPO 4 -H3. While the average Al-O and P-O bond lengths are essentially the same for AlPO 4 -H3 and AlPO 4 -C, it is worth noting that the scatter in the individual experimental values is considerably larger for the latter system (see Table of Supporting Information), where the structure was refined from powder data. A Raman spectrum of AlPO 4 -H3 for the wavenumber range from ∼2900 cm -1 to ∼3800 cm -1 was reported by Knops-Gerrits et al. . These authors identified a total of five bands, centered at 3012, 3255, 3379, 3505, and 3563 cm -1 . Figure presents a comparison of the experimental Raman spectrum to the spectrum obtained from PBE-TS calculations for the same wavenumber range, with Figure showing the unscaled calculated spectrum (for the purpose of visualisation, a Gaussian broadening of 10 cm -1 was applied to all spectra). The comparison with experiment shows a systematic shift of the PBE-TS bands to lower frequencies, however, the relative positions and intensity ratios are reproduced rather well. An excellent match between the DFT-computed and experimental Raman spectrum can be obtained by scaling the frequencies by a common factor of 1.037, as shown in Figure . A scaling factor in this range does not seem completely unreasonable for a PBE-based approach . For a free water molecule (H 2 O placed in a large box), PBE-TS gives O-H stretching frequencies of 3584 cm -1 and 3699 cm -1 , compared to experimental values of 3657 and 3756 cm -1 , and scaling by a slightly smaller factor of 1.018 would be necessary to give agreement to within 10 cm -1 for both frequencies. For the H-O-H bending mode, however, the PBE-TS frequency is slightly overestimated, with 1607 cm -1 compared to an experimental value of 1595 cm -1 . Thus, no simple scaling would lead to agreement between experiment and DFT across the whole frequency range. Since the main aim of this work is not an accurate quantitative prediction of the frequencies, but a qualitative interpretation of the modes, only unscaled IR and Raman spectra will be shown throughout. In addition to the PBE-TS calculations, further calculations for AlPO 4 -H3 and AlPO 4 -C were performed using the PBEsol-D2 functional . This functional was found to give very good agreement with experimental structure data for guest-free AlPOs and all-silica zeolites in previous work . While use of PBEsol-D2 resulted in better agreement with experimental lattice parameters than found for PBE-TS, a more pronounced tendency to underestimate the length of the hydrogen bonds was observed (see following section). Moreover, the calculated Raman spectrum agreed less well with experimental data. Therefore, the PBEsol-D2 results are not discussed any further.