Here, APC hydr and APC dehydr refer to a hydrated model and to dehydrated AlPO 4 -C, respectively. The energy for a single water molecule was obtained from a calculation for a water molecule in a box with an edge length of 15 Å. To calculate the adsorption enthalpy ∆H ads , the corresponding zero-point vibrational energy and vibrational contributions at T = 298 K, obtained from the lattice dynamical calculations, were added to each term on the right-hand side of the above equation (it has to be noted that these contributions will be of limited accuracy because only the Γ point was sampled in the calculations). The lattice dynamical calculations also delivered the anisotropic displacement parameters U ij,x for each non-equivalent atom x. The isotropic displacement parameters U iso were calculated from these values using the well-established approximation U iso,x = (U 11,x + U 22,x + U 33,x )/3 . DFT-based ab-initio molecular dynamics (AIMD) calculations were carried out for AlPO 4 -H3, using a 1 × 2 × 2 supercell of the crystallographic unit cell. The AIMD simulations were performed using the CP2K code (version 2.6.2, installed on the HLRN supercomputer "Konrad"), which employs a hybrid Gaussian and plane wave scheme . Due to unavailability of the PBE-TS functional in CP2K, the PBE functional was used in conjunction with the Grimme-type D3 dispersion correction . Furthermore, these calculations used a plane wave energy cutoff of 500 Ry, Goedecker-Teter-Hutter pseudopotentials devised by Krack , and a DZVP-MOLOPT basis set. Only the Γ point was used to sample the first Brillouin zone. The AIMD simulations were performed in the canonical (NVT) ensemble for a temperature of 298 K, using a Nosé-Hoover thermostat, a timestep of 0.5 fs and a time constant of 50 fs. The full MD trajectory consisted of 20,000 steps (10 ps), of which the first 5,000 steps were used for equilibration. The analysis of the "production" part of the trajectory was carried out with the VMD software, version 1.9.3 . From the root mean square displacements (RMSDs) calculated with VMD, the isotropic displacement parameters were calculated as U iso,x = 1 3 (RMSD x ) 2 .