Using a fully flexible molecular potential in equilibrium molecular dynamics simulations, we study the α- and γ-polymorphs of the energetic molecular crystal hexahydro-1,3,5-trinitro-s-triazine (RDX), their respective properties, and the conditions that contribute to the stress-induced γ → α solid-solid phase transition mechanisms. We find the pressure-dependent atomic structure, mechanical properties, and transition behavior to be described reasonably well. Uniaxial deformation of α-RDX along the crystal axes is shown to result in three different crystal responses where compression of the c-axis results in the α → γ transition, compression of the b-axis causes a transition with resulting structure similar to stacking faults observed by Cawkwell et al. [ J. Appl. Phys. 2010, 107, 063512], and no transitions are observed for compression of the a-axis.

Nanoindentation studies of the mechanical properties of sufficiently thin polymer films, supported by stiff substrates, indicate that the mechanical moduli are generally higher than those of the bulk. This enhancement of the effective modulus, in the thickness range of few hundred nanometers, is indicated to be associated with the propagation and impingement of the indentation tip induced stress field with the rigid underlying substrate; this is the so-called "substrate effect". This behavior has been rationalized completely in terms of the moduli and Poisson's ratios of the individual components, for the systems investigated thus far. Here we show that for thin supported polymer films, in general, information regarding the local chain stiffness and local vibrational constants of the polymers provides an appropriate rationalization of the overall mechanical response of polymers of differing chemical structures and polymer-substrate interactions. Our study should provide impetus for atomistic simulations that carefully account for the role of intermolecular interactions on the mechanical response of supported polymer thin films.

The ability of particle-based coarse-grain potentials, derived using the recently proposed multiscale coarse-graining (MS-CG) methodology [S. Izvekov and G. A. Voth, J. Phys. Chem. B 109, 2469 (2005); J. Chem. Phys. 123, 134105 (2005)] to reconstruct atomistic free-energy surfaces in coarse-grain coordinates is discussed. The MS-CG method is based on force-matching generalized forces associated with the coarse-grain coordinates. In this work, we show that the MS-CG method recovers only part of the atomistic free-energy landscape in the coarse-grain coordinates (termed the potential of mean force contribution). The portion of the atomistic free-energy landscape that is left out in the MS-CG procedure contributes to a pressure difference between atomistic and coarse-grain ensembles. Employing one- and two-site coarse-graining of nitromethane as worked examples, we discuss the virial and compressibility constraints to incorporate a pressure correction interaction into the MS-CG potentials and improve performance at different densities. The nature of the pressure correction interaction is elucidated and compared with those used in structure-based coarse-graining. As pairwise approximations to the atomistic free-energy, the MS-CG potentials naturally depend on the variables describing a thermodynamic state, such as temperature and density. Such dependencies limit state-point transferability. For nitromethane, the one- and two-site MS-CG potentials appear to be transferable across a broad range of temperatures. In particular, the two-site models, which are matched to low and ambient temperature liquid states, perform well in simulations of the ambient crystal structure. In contrast, the transferability of the MS-CG models of nitromethane across different densities is found to be problematic. To achieve better state-point transferability, density dependent MS-CG potentials are introduced and their performance is examined in simulations of nitromethane under various thermodynamic conditions, including shocked states.

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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.

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We evaluate whether lattice or internal phonons dominate the thermal excitation of the N-N bonds in α-cyclotrimethylene trinitramine (α-RDX) by computing the fractional contributions of phonon modes to the excitation of all atomic interactions. We derive a method to compute these contributions, which we call mode energy fractions, from the phonon eigenvectors and a splitting of the dynamical matrix. This enables identification of phonon modes that most strongly excite the N-N bonds that play a key role in molecular decomposition of α-RDX. Correlating these fractions with the mode populations and contributions to the specific heat and thermal conductivity, we analyze how thermal energy is distributed by phonons following a passing shock. Contrary to the common explanation that thermal energy is transferred to the N-N bonds indirectly, by internal phonons, we find that lattice phonons dominate this thermal energy transfer, implying that energy flow follows a direct route. We also comment on implications of these results for non-shock decomposition of α-RDX.

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