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 structure of the α-form of the hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX) crystal. The potential is composed of pairwise atom−atom (6-exp) Buckingham interactions and charge−charge interactions. The parameters of the Buckingham repulsion−dispersion terms have been determined through a combination of nonlinear least-squares fitting to observed crystal structures and lattice energies and trial-and-error adjustment. Crystal packing calculations were performed to determine the equilibrium crystallographic structure and lattice energy of the model. There are no significant differences in the geometrical structures and crystal energies resulting from minimization of the lattice energy with and without symmetry constraints. Further testing of the intermolecular potential has been done by performing symmetry-constrained isothermal−isobaric Monte Carlo simulations. The properties of the crystal (lattice dimensions, molecular orientation, and lattice energy) determined from Monte Carlo simulations at temperatures over the range 4.2−300 K indicate good agreement with experimental data. The intermolecular potential was also subjected to isothermal−isobaric molecular dynamics calculations at ambient pressure for temperatures ranging from 4.2 to 325 K. Crystal structures at 300 K are in outstanding agreement with experiment (within 2% of lattice dimensions and almost no rotational and translational disorder of the molecules in the unit cell). The space-group symmetry was maintained throughout the simulations. Thermal expansion coefficients were determined for the model and are in reasonable accord with experiment.