The comparative DFT calculations for partially hydrated phase clearly indicate that such phases are not stable, regardless of which water molecule is removed. The predictions are in line with the reported dehydration behaviour , and also agree with earlier calculations on CHA-type AlPO-34 . Apparently, even in the small-pore APC-type structure, Al-coordinated H 2 O molecules are stabilised only through hydrogen bonds to other water molecules adsorbed in the channels. Since the APC-type AlPO is one of the porous aluminophosphates with the smallest pores, we may expect that this finding can be generalised to all porous AlPOs, regardless of the pore size. The situation is different for dense hydrated aluminophosphates like variscite and metavariscite, and possibly also for porous AlPOs where water molecules are able to adsorb in a bridging position, simultaneously coordinating to two framework Al atoms. Such a scenario has been described for AlPO-14, where the water desorption proceeds in two steps, and where the second step was attributed to the desorption of water molecules bridging between two Al sites . A prediction of the IR and Raman spectra of AlPO 4 -H3 showed that the three non-equivalent water molecules give rise to different bands in the frequency range of O-H bond stretching. An experimental Raman spectrum has been reported for this frequency range, and the excellent agreement of frequencies (apart from a constant scaling factor) and intensities gives confidence in the computational predictions. The frequency shift with respect to a free water molecule is determined by the hydrogen bonding pattern, so information about the length/strength of the hydrogen bonds in which the adsorbed water molecules participate can be inferred from these modes. Using such complementary information from vibrational spectroscopy could help to make reasonable guesses of the hydrogen positions in cases where only the positions of the water oxygen atoms can be determined (e.g. from PXRD data). While modes in the O-H stretching frequency range cannot be used as "fingerprints" to distinguish Al-coordinated water molecules from molecules adsorbed in the pores of AlPO 4 -H3, a few "framework modes" in the frequency range below ∼1200 cm -1 that are predominantly related to deformations of AlO 6 octahedra were identified. In particular, a Raman-active mode at 308 cm -1 that seems to have a counterpart in variscite and a group of IR-active modes at ∼475 cm -1 could be modes that are indicative for the presence of octahedrally coordinated Al. In order to establish whether these modes are really useful as "fingerprints", experimental work on hydrated aluminophosphates should be performed, focussing on systems for which some information about the presence of Al-coordinated water molecules is available (e.g. from diffraction and/or NMR spectroscopy). Finally, DFT-based AIMD calculations were used to analyse the dynamic behaviour of the water molecules. Given its different bonding environment, it is not surprising that the H 2 O(pore) molecule exhibits a higher mobility than the Al-coordinated water molecules. The large mobility may also serve to explain the deviations between the position of this molecule in the DFT-optimised structure and the experimentally observed position. The Al-coordinated H 2 O(1) molecule is capable of dynamically changing its H-bonding pattern, and its behaviour is related to the mobility of H 2 O(pore): If the H 2 O(pore) molecule moves so far away that the H2• • •O11 H-bond is elongated beyond a certain extent, the O9-H2 bond reorients to form a hydrogen bond to the framework oxygen atom O6. While the dynamic behaviour of this water molecule could not be deduced from the experimental crystal structure, it has to be noted that previous AIMD studies of other hydrated AlPOs and related systems also highlighted the importance of dynamic hydrogen bonding : For example, thermal oscillations of the water molecules adsorbed in the channels of the large-pore AlPO VPI-5 cause a continuous breaking and reformation of hydrogen bonds without there being any appreciable diffusion . Another interesting observation is the good agreement of U iso values calculated using either AIMD or phonon calculations with experimental values. This indicates that U iso parameters taken from calculations could be used as realistic starting values in structure refinements, which might be especially useful in PXRD studies.