A molecular dynamics simulation study of elastic properties of HMX

Atomistic simulations were used to calculate the isothermal elastic properties for β-, α-, and δ-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The room-temperature isotherm for each polymorph was computed in the pressure interval 0⩽p⩽10.6 GPa and was used to extract the initial isothermal bulk modulus Ko and its pressure derivative using equations of state employed previously in experimental studies of the β-HMX isotherm. The complete elastic tensor for each polymorph was calculated at room temperature and atmospheric pressure. For the case of β-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 β-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 Ko 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 β-HMX should be revised from a value of ∼12.4–13.5 GPa to ∼15–16 GPa. The present report provides the first accurate determination of the elastic tensors and isotropic moduli for α- and δ-HMX. Predicted values of the shear moduli for α- and δ-HMX are more than a factor of 2 smaller than for β-HMX.