Monte Carlo calculations of the elastic moduli and pressure-volume-temperature equation of state for hexahydro-1,3,5-trinitro-1,3,5-triazine

Isothermal-isobaric Monte Carlo calculations were used to obtain prediction s of the elastic coefficients and derived engineering moduli and Poisson ra tios for crystalline hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The ela stic coefficients were computed using the strain fluctuation formula due to Rahman and Parrinello [J. Chem. Phys. 76, 2662 (1982)]. Calculations were performed as a function of temperature (218 K less than or equal to T less than or equal to 333 K) and hydrostatic pressure (0 GPa less than or equal to p less than or equal to 4 GPa). The predicted values of the moduli and P oisson ratios under ambient conditions are in accord with general expectati ons for molecular crystals and with a very recent, unpublished determinatio n for RDX. The moduli exhibit a sensitive pressure dependence whereas the P oisson ratios are relatively independent of pressure. The temperature depen dence of the moduli is comparable to the precision of the results. However, the crystal does exhibit thermal softening for most pressures. An addition al product of the calculations is information about the pressure-volume-tem perature (pVT) equation of state. We obtain near-quantitative agreement wit h experiment for the case of hydrostatic compression and reasonable, but no t quantitative, correspondence for thermal expansion. The results indicate a significant dependence of the thermal expansion coefficients on hydrostat ic pressure. (C) 2000 American Institute of Physics. [S0021-8979(00)03010-3 ].