Predicting Melt Curves of Energetic Materials Using Molecular Models

Abstract

In this work, the solid–liquid coexistence curves of classical fully flexible atomistic models of α‐RDX and β‐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 α‐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 ∼10 % of the computational resources used for 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.

Predicted Viscosity of Liquid HMX up to 40 GPa

“…Using MD simulations, we obtained direct predictions for the viscosity at pressures up to 40 GPa for a range of temperatures above the melt line predicted [33,51] using the same MD force field. These data reveal a temperature dependence that is Arrhenius at each pressure considered, which is well-fit by the empirical Andrade equation [41].…”

“…Classical, non-reactive MD simulations were performed using the LAMMPS code [43] and a variant [44] of the Smith-Bharadwaj force field (FF) for HMX and RDX [45], which has been widely validated and used to model both materials in the solid and liquid states [4,6,7,9,27,28,33,34,36,[46][47][48][49][50][51]. Simulations of liquid HMX were performed using a cubic, 3D periodic simulation cell (see Figure 1) that contained 250 molecules.…”

Predicted viscosity of liquid HMX up to 40 GPa

“…Classical, non-reactive MD simulations were performed using the LAMMPS code [43] and a variant [44] of the Smith-Bharadwaj force field (FF) for HMX and RDX [45], which has been widely validated and used to model both materials in the solid and liquid states [4,6,7,9,27,28,33,34,36,[46][47][48][49][50][51]. Simulations of liquid HMX were performed using a cubic, 3D periodic simulation cell (see Figure 1) that contained 250 molecules.…”

“…The shear viscosity of liquid HMX was determined as a function of temperature at pressures of 0, 1, 3, 5, 10, 20, 30, and 40 GPa using NVT MD simulations with fully flexible molecules. Temperatures were set in 100 K increments at values that approached the predicted melt curve from above [33,51]. A minimum of six temperature values were considered for each pressure.…”

“…Using MD simulations, we obtained direct predictions for the viscosity at pressures up to 40 GPa for a range of temperatures above the melt curve predicted [33,51] using the same MD force field. These data reveal a temperature dependence that is Arrhenius at each pressure considered, which is well-fit by the empirical Andrade equation [41].…”