Coarse-grain simulation study of the shear-band deformation mechanism in molecular crystals

Computationally inexpensive particle-based coarse-grained (CG) models are crucial for simulations of mesoscopically slow cooperative phenomena such as plastic deformations in solids. Molecular crystals possessing complex symmetry present an enormous practical challenge for coarse-graining at molecularly resolved scales, where the molecule is mapped into a CG particle and beyond. In this paper, we present the successful bottom-up coarse-graining of a molecular energetic crystal, cyclotrimethylene trinitramine in the alpha phase (α-RDX), using the force-matching based multiscale coarse-graining (a.k.a. MSCG/FM) approach.¹ The new MSCG/FM model offers a potentially powerful free-energy tool to analyze the lattice instabilities that lead to plastic response. A specific application of this model involves a study of the molecular-level mechanisms of shear microband formation, which is observed in the atomistic simulations of α-RDX under both static and shock-wave uniaxial compressions. These were hypothesized to contribute to plasticity, and potential shock initiation in RDX-based explosives.
¹ J. D. Moore, B. C. Barnes, S. Izvekov, M. Lísal, M. S. Sellers, D. E. Taylor, and J. K. Brennan, J. Chem. Phys. 144 (10), 104501 (2016).