Structure optimisations and calculations of vibrational spectra were performed with the CASTEP code (version 17.2), which uses plane waves and pseudopotentials, and which can impose constraints consistent with the space group symmetry . The core electrons were represented using norm-conserving pseudopotentials taken from the database of Bennett and Rappe () . An energy cutoff of 1000 eV was used. The calculations were carried out using the PBE exchange-correlation functional in conjunction with the dispersion correction devised by Tkatchenko and Scheffler , as the PBE-TS method was found to give fairly accurate results in previous studies of aluminophosphates . Due to the size of the unit cell of AlPO 4 -H3/AlPO 4 -C, only the Γ point was used to sample the Brillouin zone. Calculations of the vibrational properties, including IR and Raman intensities, used the density functional perturbation theory formalism as implemented in CASTEP . In order to determine the interaction strength between framework and adsorbed water molecules, both the interaction energy ∆E int and the adsorption enthalpy ∆H ads for T = 298 K were calculated for fully hydrated AlPO 4 -H3 and several partially hydrated models. The calculations of the interaction energy per water molecule made use of the DFT energies E PBE-TS obtained from the structure optimisations: