In their sc-XRD investigation, Pluth and Smith obtained an almost complete structure of AlPO 4 -H3, which includes the oxygen position of the three non-equivalent water molecules . While the hydrogen bonds formed by the Al-coordinated water molecules have already been described above, those of H 2 O(pore) can only be assessed after the DFT optimisation: Here, the H6 hydrogen atom forms a single hydrogen bond to the framework oxygen atom O4, whereas the H5 hydrogen has similarly close contacts to O1 and O8, both in the range of 2.35 Å (Figure ). This bonding scenario is best described as a "bifurcated" hydrogen bond, and is reminiscent of some hydrogen bonds present in the nonporous hydrated aluminophosphate metavariscite . For the Al-coordinated water molecules, a comparison of the DFT-optimised H• • •O distances to experimental values shows that the hydrogen bonds are consistently shorter in the PBE-TS optimised structure, with differences up to 0.36 Å. This can be attributed to the underestimation of hydrogen bond distances by PBE-based approaches, which has been demonstrated previously ,