Ab initio density functional theory (DFT) has been used to study the microporous aluminophosphate AlPO4-34 (CHA topology) in the calcined dehydrated and rehydrated forms. DFT calculations allowed us first to reproduce experimental observations obtained by X-ray diffraction, solid-state NMR, and thermal analysis and then to obtain a better understanding from dynamical simulations of the structure modifications occurring during hydration of the calcined material. The method and simulation tools were first calibrated on calcined,never-hydrated AlPO4-34. After complete relaxation, the structure was in excellent agreement with the experimental ones. A static approach of the fully hydrated material (with 12 H2O per unit cell) did not lead to a unique structure but to various polymorphs, differing essentially by the position of the water molecules. A more realistic molecular dynamic approach confirmed that at room-temperature some of the water molecules show large oscillations in the framework and even diffuse within the pores of the aluminophosphate. A combination of structure resolution by Rietveld refinement and molecular dynamics has been proposed to define an average structure and its structural behavior. From the calculated binding energies, the hydration mechanism takes place in one step without the formation of a stable intermediate phase. A structure for the partially hydrated compound (11 H2O per unit cell) has been determined on the basis of energetic, geometrical, and experimental arguments.