Periodic DFT Studies of AlPO-11:  The Role of Hydration on Structural Properties

Periodic density functional theory B3LYP calculations are performed to study the structure of AlPO-11 and the possible effects of hydration on three different crystallographic T-sites. Previously tested atomic basis sets were used, and full geometry optimizations were done within the Ibm2 space group for anhydrous and for T1-, T2-, T3-hydrated structures. A detailed comparison of the relevant geometric parameters is done for the B3LYP optimized structures with and without hydration. The most stable ideal hydration coordinates water molecules to the T1-site and induces a significant change in the b cell parameter with respect to the anhydrous case, thus leading to better agreement with the experimental b value. However, improving the atomic basis sets and correcting for basis set superposition error (BSSE) yields energy differences between the various hydration situations on the order of 1 kcal/mol. This suggests that thermal mobility of water molecules inside the large pore of AlPO-11 will play a significant role in determining the average structure.