Atomistic simulations were used to calculate the isothermal elastic properties for -, ␣-, and ␦-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine ͑HMX͒. The room-temperature isotherm for each polymorph was computed in the pressure interval 0р pр10.6 GPa and was used to extract the initial isothermal bulk modulus K o and its pressure derivative using equations of state employed previously in experimental studies of the -HMX isotherm. The complete elastic tensor for each polymorph was calculated at room temperature and atmospheric pressure. For the case of -HMX, the calculated elastic tensor is compared to one based on a fit to sound speed data yielding reasonably good agreement. The bulk modulus of -HMX obtained from equation-of-state fits to the room-temperature isotherm agrees well with that determined from the complete elastic tensor and from volume fluctuations at atmospheric pressure. However, the value of K o obtained from the isotherm is sensitive to choice of equation of state fitting form and to the weighting scheme employed in the fit. Based upon simulation results and reanalysis of experimental data, the commonly accepted value of the initial isothermal bulk modulus for -HMX should be revised from a value of ϳ12.4-13.5 GPa to ϳ15-16 GPa. The present report provides the first accurate determination of the elastic tensors and isotropic moduli for ␣and ␦-HMX. Predicted values of the shear moduli for ␣and ␦-HMX are more than a factor of 2 smaller than for -HMX.
Quantum chemistry-based dipole polarizable and nonpolarizable force fields have been developed for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Molecular dynamics simulations of TATB crystals were performed for hydrostatic pressures up to 10 GPa at 300 K and for temperatures between 200 and 400 K at atmospheric pressure. The predicted heat of sublimation and room-temperature volumetric hydrostatic compression curve were found to be in good agreement with available experimental data. The hydrostatic compression curves for individual unit cell parameters were found to be in reasonable agreement with those data. The pressure- and temperature-dependent second-order isothermal elastic tensor was determined for temperatures between 200 and 400 K at normal pressure and for pressures up to 10 GPa on the 300 K isotherm. Simulations indicate considerable anisotropy in the mechanical response, with modest softening and significant stiffening of the crystal with increased temperature and pressure, respectively. For most properties the polarizable potential was found to yield better agreement with available experimental properties.
W91INF-05-1-0265 REPORT NUMBER REPORT ~'u~mER 48101-EG-MUR The views, opinions andlor findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other documentation. Approved for public release; federal purpose rights The response of the energetic molecular crystal cyclotrimethylene trinitramine (RDX) to the propagation of planar shock waves nonnal to (100) has been studied using large-scale molecular dynamics simulations that employ an accurate and transferable nonreactive potential. The propagation of the shock waves was simulated using nonequilibrium molecular dynamics. Shear bands were nucleated during shocks with a particle velocity of 1.0 km s-I and corresponding Rankine-Hugoniot shock pressun of9.7 GPa. These defects propagate into the compressed material at 45° to (100) in the (010) zone. The shear bands e\'oh'e slowly compared to the time scales routinely accessible to nonequilibrium molecular dynamics toward a liquidlike state as a result of viscous heating. A recently developed shock-front absorbing boundary condition [AV. Bolesta et al. , Phys. Rev. B 76, 224108 (2007)] was applied to the simulation cells at the moment of maximum compression to sustain the shock-compressed state. Molecular dynamics simulations were then employed to study the temIXlral and structural evolution of the shock-induced shear bands toward a steady-fluctuating state. Owing to the intense, viscous flow-driven heati ng within the shear bands. these defects can be considered to be homogeneously nucleated hot spots.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.