Predicting Melt Curves of Energetic Materials Using Molecular Models

In this work, the solid-liquid coexistence curves of classical fully flexible atomistic models of alpha-RDX and beta-HMX were 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, which were then fit to the Simon-Glatzel functional form. While effective, this method is computationally expensive. An alternative approach is to compute the melting point at a single pressure via the PSCP method, and then use the Gibbs-Duhem integration technique to trace out the coexistence curve in a more computationally economical manner. Both approaches were used to determine the coexistence curve of alpha-RDX. The Gibbs-Duhem integration method was shown to generate a melt curve that is in good agreement with the PSCP-derived melt curve, while only costing similar to 10 % of the computational resources used for the PSCP method. For alpha-RDX, the predicted melting temperature increases significantly more for a given increase in pressure when compared to available experimental data.