Computationally inexpensive particle-based coarse-grained (CG) models are crucial for simulations of mesoscopically slow cooperative phenomena such as plastic deformations in solids. To date, no pairwise models have been reported in the literature that are able to reproduce complex crystallographic symmetries of molecular crystals. 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, which implements a pairwise decomposition of the crystalline potential of mean force (PMF) in molecular center-of-mass (COM) coordinates, 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 mechanism for nanoscale shear bands, which are observed in the atomistic simulations of α-RDX under static and shock-wave uniaxial compressions. These were hypothesized to contribute to plasticity, heat localization, and potential shock initiation in RDX-based explosives. Atomistic and CG molecular dynamics simulations of static compression reveal a cooperative mechanism for nanoscale shear strain localization, where the plastic event begins with transitioning from an affine lattice distortion to a martensitic phase separation into tetragonal domains of different phases. The CG simulations indicate that the modification of the effective intermolecular interaction under compression are a cause for the stress anisotropy and subsequent strain localization.
[1] J. D. Moore, B. C. Barnes, S. Izvekov, M. Lísal, M. S. Sellers, D. E. Taylor, and J. K. Brennan, “A coarse-grain force field for RDX: Density dependent and energy conserving”, J. Chem. Phys. 144 (10), 104501 (2016).
