Derivation of force field parameters, and force field and quantum mechanical studies of layered alpha- and gamma-zirconium phosphates

A set of force field (FF) parameters was derived for bonds and atoms of the uncommon structural units (Zr-O; O-Zr-O; Zr-O-P; etc.) in layered alpha- a nd gamma-zirconium phosphates (alpha- and gamma-ZrP). To accomplish this pa rametrization we relied on the technique of energy derivatives obtained fro m ab initio quantum mechanics on a model compound as outlined by Dinur and Hagler. To check the reliability of the derived FF parameters the crystal s tructures of alpha- and gamma-ZrP were calculated using the Open Force Fiel d routine of Cerius(2). The computed results were compared with the experim ental X-ray crystal structures and also with the energy results obtained fr om the CRYSTAL95 program that performs quantum mechanical calculations on p eriodic systems. The discrepancies between the FF optimized structures and the experimental structures for alpha- and gamma-ZrP were quite acceptable. The unit cells attained differences smaller than 6% (beta angle of the gam ma-ZrP cell), while for the valence coordinate the maximum root-mean-square deviation values were 0.03 Angstrom for the bond distances (P-O in gamma-Z rP), and 5.82 degrees for the bond angles (Zr-O-P in alpha-ZrP). The CRYSTA L95 calculated energies (per zirconium phosphate unit) using as input the e xperimental X-ray structure, and the FF optimized geometry, showed small di fferences. The Delta E values (1.26 and 3.26 kcal mol(-1), for alpha- and g amma-ZrP, respectively, in favor of the X-ray geometry) does not rule out t he population of both experimental and FF structures. Furthermore, their ca lculated electronic characteristics were analogous. Finally, comparisons ma de using vibrational spectroscopy data as benchmarks showed that calculated vibrational bands were in acceptable agreement with experiments.