Thermal conductivity tensor of β-HMX as a function of pressure and temperature from equilibrium molecular dynamics simulations

We apply the Green–Kubo (G-K) approach to obtain the thermal conductivity tensor of β-1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (β-HMX) as a function of pressure and temperature from equilibrium molecular dynamics (MD) simulations. Direct application of the G-K formula exhibits slow convergence of the integrated thermal conductivity values even for long (120 ns) simulation times. To partially mitigate this slow convergence, we developed a numerical procedure that involves physically justified filtering of the MD-calculated heat current to remove contributions which do not contribute to the conductivity tensor. Also, by combining keyword options in LAMMPS, which was used as the MD engine, we obtained a “hybrid” heat current that is physically more realistic for β-HMX than those predicted by either keyword alone. The heat-current correlation functions from the filtered hybrid heat current exhibit significantly reduced oscillations and lead to a much smoother behavior of the G-K time integrals. A physically motivated double-exponential function was fitted to the integrated time-dependent thermal conductivity tensor components to obtain the asymptotic values for the tensor. This procedure was used to determine the thermal conductivity tensor of β-HMX as a function of pressure and temperature for 0 ≤ ?? ≤ 30 GPa and 300 K ≤ ?? ≤ 900 K. The thermal conductivity increases with increasing ??, by approximately an order of magnitude over the interval considered, and decreases with increasing temperature. The predictions are compared to experimental and other theoretically determined values for the thermal conductivity.