A CONSISTENT MOLECULAR MECHANICS FORCE-FIELD FOR ALUMINOPHOSPHATES

A consistent molecular mechanics force field has been developed for aluminophosphate structures including berlinite and several molecular sieve structures, which predicts the geometry, heat of formation and the vibrational frequencies reasonably well. The force field consists of a bonding term, a bending term and long-range interactions. Charges are calculated using the electronegativity equalization method of Mortier et al. (W. J. Mortier, S. K. Ghoshand and S. Shankar, J. Am. Chem. Soc., 1986,108, 4315), modified with the relative permittivity. The parameters for oxygen are transferable with the all-silica force field recently developed. Results are given for berlinite, AlPO-5 (AFI), AlPO-8 (AET), AlPO-11 (AEL), AlPO-18 (AEI), AlPO-25 (ATV), AlPO-35 (LEV), AlPO-37 (FAU) and VPI-5 (VFI), as well as for the hydrated structures of variscite and VPI-5 (six-fold coordination around aluminium) and the structures of AlPO-18 and AlPO-21 (ATO) with extra-framework oxygen (five-fold coordinated aluminium). Atomic positions of the AlPO-35 structure (both DLS and energy minimized), which have not been reported before, are also given.