The COMPASS force field: parameterization and validation for phosphazenes

A new, condensed-phase optimised ab-initio force field, COMPASS, has been developed recently. In this paper, the validation of COMPASS for phosphazenes is presented. The functional forms of this force field are of the consistent force field (CFF) type. Charges and bonded terms were derived from HF/6–31G∗ calculations, while the nonbonded parameters (L-J 9-6 vdW potential) were initially transferred from the polymer consistent force field, pcff, and optimised using MD simulations of condensed-phase properties. As a validation of COMPASS, molecular mechanics calculations and molecular dynamics simulations have been made on a number of isolated molecules, liquids, and crystals. The calculated molecular structure, vibration frequencies, conformational properties for isolated molecules, crystal cell parameters and density, liquid density, and heat of evaporation agreed favourably with most experimental data. The special conformational properties of the tetracyclophosphazenes, (NPCI2)4 and (NPF2)4, in the solid state are discussed based on molecular mechanics and CASTEP ab-initio calculations. The effect of nonbonded cutoff distance and different algorithms for pressure control in NPT simulation was also investigated. Finally, molecular dynamics using the COMPASS force field was used to predict properties of three isomers of high-molecular-weight amorphous poly(dibutoxyphosphazenes). In this case, excellent agreement was achieved between densities and glass transition temperatures obtained from dynamics and experimental data.