Ab initio and molecular dynamics studies of solid -HMX: effects of hydrostatic pressure and high temperature

Using first-principles density functional theory and classical molecular dynamics (MD), the structural, electronic and mechanical properties of the energetic material -HMX have been studied. The crystal structure optimised by the local density approximation calculations compares reasonably with the experimental data. Electronic band structure and density of states indicate that -HMX is an insulator with a band gap of 3.059eV. The pressure effect on the crystal structure and physical properties has been investigated in the range of 0-40GPa. The crystal structure and electronic properties change slightly as the pressure increases from 0 to 2.5GPa; when the pressure is above 2.5GPa, further increment of the pressure results in significant changes in crystal structure. There is a larger compression along the b-axis than along the a- and c-axes. Isothermal-isobaric MD simulations on -HMX were performed in the temperature range of 5-400K. Phase transition at 360K, corresponding to a volume interrupt, was found. The computed thermal expansion coefficients show anisotropic behaviour with a slightly larger expansion along the b- and c-axes than along the a-axis. In the temperature range of 5-360K, -HMX possesses good plasticity and its stiffness decreases with increasing the temperature.