Particle-based multiscale coarse graining with density-dependent potentials: Application to molecular crystals (hexahydro-1,3,5-trinitro-s-triazine)

Abstract

We describe the development of isotropic particle-based coarse-grain models for crystalline hexahydro-1,3,5-trinitro-s-triazine (RDX). The coarse graining employs the recently proposed multiscale coarse-graining (MS-CG) method, which is a particle-based force-matching approach for deriving free-energy effective interaction potentials. Though one-site and four-site coarse-grain (CG) models were parameterized from atomistic simulations of non-ordered (molten and ambient temperature amorphous) systems, the focus of the paper is a detailed study of the one-site model with a brief recourse to the four-site model. To improve the ability of the one-site model to be applied to crystalline phases at various pressures, it was found necessary to include explicit dependence on a particle density, and a new theory of local density-dependent MS-CG potentials is subsequently presented. The density-dependency is implemented through interpolation of MS-CG force fields derived at a preselected set of reference densities. The computationally economical procedure for obtaining the reference force fields starting from the interaction at ambient density is also described. The one-site MS-CG model adequately describes the atomistic lattice structure of α-RDX at ambient and high pressures, elastic and vibrational properties, pressure-volume curve up to P = 10 GPa, and the melting temperature. In the molten state, the model reproduces the correct pair structure at different pressures as well as higher order correlations. The potential of the MS-CG model is further evaluated in simulations of shocked crystalline RDX.

Multiscale modeling of energetic materials: Easy to say, harder to do

“…We are pursuing a bottoms-up method to develop these potentials (known as MS-CG), which effectively homogenizes atomistic information obtained from either QMD or classical MD into a particle based CG model. The details of this methodology and applications to nitromethane and RDX are detailed in recent publications [11,12]. In these, we have shown that CG MD results for structural, vibrational and elastic properties are in good agreement with atomistic simulation results for these systems, as well as agreement of shock properties, thus validating proper homogenization.…”

A perspective on modeling the multiscale response of energetic materials

“…3 shows a superposition of the temperature profiles of shocked RDX at a single point in time for three simulations, as well as the corresponding snapshot of the molecular centers of mass positions from the atomistic simulation. These correspond to MD simulations using the atomistic model 7 , MD simulations using a CG model of RDX 13 , and DPD-E simulations using the same CG model of RDX. The positions of the shock fronts for the CG simulations are slightly behind that of the atomistic-MD simulations; note that the CG-MD simulation has a peak shock temperature that is 3,000 K higher than the atomistic-MD model (which melts the material).…”

Shock response of single crystal and nanocrystalline pentaerythritol tetranitrate: Implications to hotspot formation in energetic materials

“…As a result, the exact values of temperatures may be affected. 23 Thus, the temperature values should not be taken literally, and rather represent a trend in a semi-quantitative way. A direct comparison of all-atom simulations with some of the CG-MD simulations will be done in the future.…”

“…In this article, we report MD simulations of shock response of single crystal and nanocrystalline PETN using a CG model, which reduces computational cost while retaining reasonable accuracy, 20 necessary for simulations of large polycrystalline systems. Recently, Arman et al 51 applied a CG-MD shock simulations to polymers, and Izvekov et al 23 performed multiscale CG simulations on RDX. Our objective is to characterize in atomic detail the mechanical deformation at GBs and in grain interiors, and make connections to hotspot formation.…”

“…5-10 a) wuha@ustc.edu.cn b) sluo@pims.ac.cn Theoretical or simulation studies on condensed-phase energetic materials employ such techniques as static electronic structure, [11][12][13][14] molecular dynamics (MD) simulations using empirical or electronic-structure-based force fields, [15][16][17] as well as reactive force fields (ReaxFF), 18,19 and ones based on coarse-grained (CG), particle-based methods. [20][21][22][23] The energetic materials explored include nitromethane, 11,15,16,[24][25][26][27][28][29] octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), [30][31][32][33][34] hexa-hydro-1,3,5-trinitro-1,3,5-triazine (RDX), 18,[35][36][37][38][39][40][41][42] 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), 12,13 and pentaerythritol tetranitrate (PETN), 23 among others. 14 See Rice and Sewell 43 for a recent review on this subject.…”