An analysis of the DFT-computed IR and Raman spectra was performed with emphasis on two different aspects: On the one hand, it was analysed which water molecules are responsible for which bands in the frequency region above 1200 cm -1 (O-H stretching, H-O-H bending) in order to test whether vibrational spectroscopy allows for a distinction between Al-coordinated molecules and water molecules adsorbed in the channels. On the other hand, it was attempted to identify lower-frequency modes (< 1200 cm -1 ) that could potentially be used as "fingerprint" modes indicative for the presence of six-coordinated Al atoms. The analysis proceeded as follows: After calculating IR and Raman spectra, the displacement patterns associated with all modes having significant intensity were visualised using Jmol in order to get a qualitative overview of the most prominent displacements. In the following, the magnitude of the atomic displacements was analysed in a more quantitative fashion to determine which atoms make the largest contribution to certain modes of particular interest. Clearly, any such assignment remains a simplification, because every mode is associated with the displacement of several atoms. Nevertheless, it was often possible to identify one or a few non-equivalent atom(s) that predominantly contribute to a given mode. The Supporting Information contains the CASTEP *.phonon files of AlPO 4 -H3 and AlPO 4 -C, from which the atomic displacements can be visualised with Jmol. The first part of this analysis deals with the O-H stretching region between ∼2900 and 3600 cm -1 . The IR and Raman spectra calculated for this frequency region are shown in Figure . All bands are shifted towards lower frequencies (red-shifted) with respect to free water. This red-shift is a consequence of the elongation of the covalent O-H bonds caused by the involvement of the water molecules in hydrogen bonds . The assignment of displacements of the non-equivalent hydrogen atoms to different modes is summarised in Table , together with the O-H and H• • •O distances for each H atom. Starting from the high-frequency end of the spectrum, the bands at ∼3500 cm -1 are predominantly associated with displacements of the H5 atom, which participates in the bifurcated hydrogen bond, having the shortest O-H bond length (i.e. least elongation compared to free water) and the longest H• • •O distances. The bands between ∼3380 and 3450 cm -1 are primarily due to vibrations of the H1 and H6 atoms. Although these hydrogen atoms belong to different types of water molecules (H6: H 2 O(pore), H1: Al-coordinated H 2 O(1)), they contribute to bands in the same frequency range because of similar hydrogen bond distances and, correspondingly, almost identical O-H bond lengths. The correlation between shorter hydrogen bonds (= more elongated O-H bonds) and a red-shift of the stretching frequency also holds for the other hydrogen atoms: H2 and H3 participate in hydrogen bonds with a length of roughly 1.8 Å, and they both contribute to bands between 3140 and 3190 cm -1 , with some modes having essentially equal contribution from both hydrogen atoms. Finally, the hydrogen atom H4, which forms a very short hydrogen bond of ∼1.6 Å, gives rise to strongly red-shifted bands below 3000 cm -1 . In their experimental study, Knops-Gerrits et al. assigned the two rather diffuse bands at lower frequencies to Al-coordinated water molecules, and the sharper, intense bands at the highest frequencies to water molecules in the channels (Figure shows the experimental data) . Although this assignment is largely confirmed by the calculations, we have to note that the hydrogen atom H1, despite being part of an Al-coordinated water molecule, contributes to high-frequency bands because it participates in a relatively long hydrogen bond. In the frequency range of H-O-H bending vibrations, two main bands are visible in the IR and Raman spectra, shown in Figure . Both bands are comprised of contributions from several individual modes and are therefore relatively broad. The lower-frequency band extends from about 1600 to 1630 cm -1 , and is thus close to the frequency of the H-O-H bending mode in a free water molecule (1607 cm -1 in PBE-TS calculations). The other band is blue-shifted with respect to free water, extending from about 1670 to 1700 cm -1 . An analysis of the atomic displacements shows that the lower-frequency band arises from both H 2 O(pore) and H 2 O(1): The hydrogen atoms H5 and H6 of the H 2 O(pore) molecule contribute mainly to modes between 1606 and 1611 cm -1 , whereas H1 and H2 contribute to modes between 1619 and 1625 cm -1 . Consequently, the higher-frequency band is predominantly associated with displacements of the H3 and H4 atoms, which belong to the H 2 O(2) molecule. As shown in Table , there is no correlation between the frequency and the α(H-O-H) angle. The differences in the frequencies of the H-O-H bending modes are primarily related to the length (and thus, strength) of the hydrogen bonds in which the water molecules participate: The two strong hydrogen bonds of H 2 O(2) lead to an increase in energy (and hence frequency) of the H-O-H bending deformation, whereas the longer, weaker hydrogen bonds in which H 2 O(1) and H 2 O(pore) participate have only a modest effect. Taking together the observations made for O-H stretching and H-O-H bending modes, we can conclude that their frequencies are determined by the hydrogen bonding environment of the water molecules. Because the hydrogen bonding patterns may be different for non-equivalent Al-coordinated water molecules (as is the case for H 2 O(1) and H 2 O(2)), or rather similar for Al-coordinated water molecules and molecules adsorbed in the pores, the modes in this frequency range are no reliable indicator to distinguish between water molecules in different environments. Finally, we turn our attention to the "framework modes", i.e. all modes up to ∼1200 cm -1 . The IR and Raman spectra of fully hydrated AlPO 4 -H3 and calcined AlPO 4 -C are shown in Figure . It is worth noting that the spectra display some common features, but also various differences, which are too numerous to discuss individually. Therefore, the present analysis focusses on a few selected bands that have significant intensity in the hydrated phase, but are absent or very weak in the calcined phase. One such band is found in the Raman spectrum at a frequency of 308 cm -1 . It is associated primarily with displacements of the octahedrally coordinated Al2 atom and the attached oxygen atoms, i.e. deformations of the AlO 6 octahedra. It is worth noting that an intense band at 326 cm -1 in the Raman spectrum of variscite was attributed to Al-O stretching vibrations . Variscite is a hydrated aluminophosphate with composition AlPO 4 •2H 2 O that contains only octahedrally coordinated aluminium atoms. At frequencies around 475 cm -1 , there are a few modes that give rise to a prominent band in the IR spectrum of AlPO 4 -H3 that has no direct equivalent in AlPO 4 -C. These modes are associated with a displacement of Al2 from the centre of the AlO 6 octahedron, corresponding to a combination of Al-O bond stretching and O-Al-O angle bending. Another IR-active mode at 521 cm -1 is related to an off-centre displacement of the Al2 atom within the octahedron, but also has a strong contribution from the H 2 O(1) molecule.