Theoretical studies of the hydrostatic compression of RDX, HMX, HNIW, and PETN crystals

A previously developed intermolecular potential for nitramines and several other classes of nitro-compound crystals has been used to investigate the behavior of the energetic materials hexahydro-1,3,5-trinitro-1,3,5s-triazine (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), 2,4,6,8,10,12-hexanitrohexaaza-isowurtzitane (HNIW), and pentaerythritol tetranitrate (PETN) under hydrostatic compression. Isothermal-isobaric molecular simulations (assuming the rigid-molecule approximation) and molecular packing calculations were used to perform the analyses. In the case of the RDX, HMX, and HNIW crystals, the results indicate that the proposed potential model is able to reproduce accurately the changes in the structural crystallographic parameters as functions of pressure for the entire range of pressures that has been investigated experimentally. In addition, the calculated bulk moduli of RDX and HMX were found to be in good agreement with the corresponding experimental results. In the case of the PETN crystal, the crystallographic parameters have been reproduced with an acceptable accuracy at pressures up to about 5 Cpa. The larger deviations from the experimental results at greater pressures indicate the limitations of the rigid-molecule model when applied to floppy molecules. The similarity of the results determined in molecular packing calculations relative to those from molecular dynamics simulations suggest that the former method can be used as an efficient tool for rapid tests of the crystal structure modification under pressure.