INTERMOLECULAR POTENTIAL FOR THE HEXAHYDRO-1,3,5-TRINITRO-1,3,5-S-TRIAZINE CRYSTAL (RDX) - A CRYSTAL PACKING, MONTE-CARLO, AND MOLECULAR-DYNAMICS STUDY

We have developed an intermolecular potential that describes the struc ture of the alpha-form of the hexahydro-1,3,5-trinitro-1,3,5-s-triazin e (RDX) crystal. The potential is composed of pairwise atom-atom (6-ex p) Buckingham interactions and charge-charge interactions. The paramet ers of the Buckingham repulsion-dispersion terms have been determined through a combination of nonlinear least-squares fitting to observed c rystal structures and lattice energies and trial-and-error adjustment. Crystal packing calculations were performed to determine the equilibr ium crystallographic structure and lattice energy of the model. Then a re no significant differences in the geometrical structures and crysta l energies resulting from minimization of the lattice energy with and without symmetry constraints. Further testing of the intermolecular po tential has been done by performing symmetry-constrained isothermal-is obaric Monte Carlo simulations. The properties of the crystal (lattice dimensions, molecular orientation, and lattice energy) determined fro m Monte Carlo simulations at temperatures over the range 4.2-300 K ind icate good agreement with experimental data. The intermolecular potent ial was also subjected to isothermal-isobaric molecular dynamics calcu lations at ambient pressure for temperatures ranging from 4.2 to 325 K . Crystal structures at 300 K are in outstanding agreement with experi ment (within 2% of lattice dimensions and almost no rotational and tra nslational disorder of the molecules in the unit cell). The space-grou p symmetry was maintained throughout the simulations. Thermal expansio n coefficients were determined for the model and are in reasonable acc ord with experiment.