Shockwave interactions with material microstructure localizes energy into hotspots, which act as nucleation sites for complex processes such as phase transformations and chemical reactions. To date, hotspots have been described via their temperature fields. Nonreactive, all-atom molecular dynamics simulations of shock-induced pore collapse in a molecular crystal show that more energy is localized as potential energy (PE) than can be inferred from the temperature field and that PE localization persists through thermal diffusion. The origin of the PE hotspot is traced to large intra-molecular strains, storing energy in modes readily available for chemical decomposition.

The response of high-energy-density materials to thermal or mechanical insults involves coupled thermal, mechanical, and chemical processes with disparate temporal and spatial scales that no single model can capture. Therefore, we developed a multiscale model for 1,3,5-trinitro-1,3,5triazinane, RDX, where a continuum description is informed by reactive and nonreactive molecular dynamics (MD) simulations to describe chemical reactions and thermal transport. Reactive MD simulations under homogeneous isothermal and adiabatic conditions are used to develop a reduced-order chemical kinetics model. Coarse graining is done using unsupervised learning via non-negative matrix factorization. Importantly, the components resulting from the analysis can be interpreted as reactants, intermediates, and products, which allows us to write kinetics equations for their evolution. The kinetics parameters are obtained from isothermal MD simulations over a wide temperature range, 1200−3000 K, and the heat evolved is calibrated from adiabatic simulations. We validate the continuum model against MD simulations by comparing the evolution of a cylindrical hotspot 10 nm in diameter. We find excellent agreement in the time evolution of the hotspot temperature fields both in cases where quenching is observed and at higher temperatures for which the hotspot transitions into a deflagration wave. The validated continuum model is then used to assess the criticality of hotspots involving scales beyond the reach of atomistic simulations that are relevant to detonation initiation.

The shock to detonation transition in heterogeneous high energy density solids starts with the spatial localization of mechanical energy into so-called hotspots that form due to the interaction between the leading wave and microstructural features and defects. We used large-scale molecular dynamics to characterize the hotspots resulting from the shock-induced collapse of cylindrical voids and elongated cracks focusing on the effect of shock strength, defect shape, and size. The temperature fields resulting from the collapse of cracks elongated along the shock direction show significantly higher sensitivity to both shock strength and size than cylindrical voids. Cracks 80 nm in length result in temperatures almost three times higher than voids 80 nm in diameter, reaching values corresponding to the ideal case of isentropic recompression of a gas. The molecular dynamics trajectories reveal the atomic origin of this contrasting behavior. While circular voids undergo a transition from viscoelastic pore collapse to a hydrodynamic regime with increasing shock strength, shock focusing in elongated cracks results in jetting and vaporization which, upon recompression, leads to increased heating.

Approximating the dynamics of atomic nuclei with classical equations of motion in molecular dynamics (MD) simulations causes an overprediction of the specific heat and omits zero-point energy which can have a significant effect on predictions of the response of materials under dynamical loading. We use quantum and classical thermostats in reactive MD simulations to characterize the effect of energy distribution on the initiation and decomposition of the explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under shock and thermal loading. Shock simulations using the multiscale shock technique (MSST) show that nuclear quantum effects not only increase the temperature rise during dynamical loading but also lower the shock temperature corresponding to the threshold for initiation of chemical reactions. The lower specific heat and presence of zero point energy contribute approximately equally to these effects. Thermal decomposition simulations show that nuclear quantum effects lower the activation barrier associated with reaction compared to classical simulations. Quite interestingly, comparing quantum and classical simulations as a function of average kinetic energy shows that classical baths result in faster kinetics as compared with quantum ones; we explore the molecular origins of this observation.

Mechanical forces acting on atoms or molecular groups can alter chemical kinetics and decomposition paths. So called mechanochemistry has been proposed to influence a variety of processes, from the formation of prebiotic compounds during planetary collisions to the shock-induced initiation of explosives. It has also been harnessed in various engineering applications such as mechanophores and ball milling in industrial applications. Experimental and computational tools designed to characterize the effect of mechanics on chemistry have focused exclusively on simple linear forces between pairs of atoms or molecular groups. However, the mechanical loading in condensed matter systems is significantly more complex and involves many-body deformations. Therefore, we propose a methodology to characterize the effect of many-body intra-molecular strains on decomposition kinetics and reaction pathways. We combine four-body external potentials with reactive molecular dynamics and show that many body strains that mimic those observed in condensed matter encourage bond rupture in a spiropyran mechanophore and accelerate thermal decomposition of condensed TATB, an energetic material. The approach is generalizable to a variety of systems and can be used in conjunction with ab initio molecular dynamics, and the two examples utilized here illustrates both the versatility of the method and the importance of many-body mechanochemistry.