Melt Curves of RDX and HMX Computed by Molecular Simulation

In this talk, we show how the solid–liquid coexistence curves of classical fully flexible atomistic models of α-RDX and β-HMX can be calculated using thermodynamically rigorous methodologies that identify where the free energy difference between the phases is zero. The free energy difference between each phase at a given state point was computed using the pseudosupercritical path (PSCP) method, and Gibbs–Helmholtz integration was used to evaluate the solid–liquid free energy difference as a function of temperature. This procedure was repeated for several pressures to determine points along the coexistence curve. While effective, this method is computationally expensive. To trace out the coexistence curve in a more computationally economical manner, Gibbs–Duhem integration was used starting from a coexistence point determined by the PSCP method. For α-RDX, the predicted melting temperature increases significantly more for a given increase in pressure when compared to available experimental data.