The starting structure for the DFT optimisation of AlPO 4 -H3 was taken from the work of Pluth and Smith , the only modification being the addition of two hydrogen atoms (H5 and H6) to the O11 atom of the H 2 O(pore) molecule. These atoms were added in a way that the angles between the H1• • •O11 and H2• • •O11 hydrogen bonds and the O11-H5 and O11-H6 bonds correspond approximately to the tetrahedral angle. Partially hydrated models were constructed by starting from the PBE-TS optimised structure of AlPO 4 -H3 and removing either one or two water molecules, leading to a total of six different partially hydrated models. The structure of fully dehydrated AlPO 4 -C was also optimised. Here, the initial structure was taken from the PXRD study by Keller, Meier, and Kirchner .