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Ultra-fast Chemistry Under Non-equilibrium Conditions and the Shock to Deflagration Transition at the Nanoscale

Summary

Reactive molecular dynamics with ReaxFF in LAMMPS follows shock-induced collapse of cylindrical voids in RDX, focusing on ultrafast, non-equilibrium chemistry and the emergence of a deflagration front from a nanoscale hot spot. The study contrasts dynamically loaded hot spots with a static thermal hot spot of comparable conditions. Energetic material safety models often assume thermal explosion kinetics; this work asks whether shock focusing at voids produces distinct reaction pathways and front structures not captured by static heating alone.

Methods

MD application (atomistic dynamics)

  • Engine / code: LAMMPS with ReaxFF for α-RDX chemistry; the parameterization merges the authors’ nitramine training line with the combustion-branch ReaxFF data as cited in the manuscript.
  • System construction: supported shock samples are built from a perfect α-RDX crystal containing a single cylindrical void; pore diameters 10–40 nm are studied by scaling supercell replications (the 40 nm case is built from an 8-molecule unit cell replicated 84×3×204 along [100], [010], [001] with the pore centered 60 nm from the impact face; smaller pores use scaled replications as tabulated in Section 2.1).
  • Pre-shock conditioning: energy minimization, then 300 K equilibration: 5 ps isobaric-isothermal followed by 10 ps isochoric-isothermal (as stated in Section 2.1).
  • Electrostatics / ReaxFF charge solve: self-consistent partial charges are updated each 0.1 fs time step using a conjugate-gradient solver to a tolerance 1×10⁻⁶ (as stated in Section 2.2).
  • Shock / ensemble for the shock segment: a momentum mirror piston imparts a particle velocity of 2 km s⁻¹, producing ~11 GPa shock pressure in the overdriven regime described in the text; the shocked evolution is propagated in constant-energy MD (NVE) after piston impact (Section describing shock setup in the PDF).
  • Diagnostics: spatially resolved temperature, composition, and reaction-zone tracking through void collapse and subsequent chemistry.
  • Barostat during shock: N/A — shock segment is NVE piston-driven rather than stochastic NPT.
  • Electric field / replica sampling: N/A — not used.
  • Boundary conditions / thermostat: samples use 3D periodic boundary conditions (PBC) for the crystalline RDX supercells; the shocked segment is NVE without an active Nose–Hoover or Berendsen thermostat during the momentum-mirror propagation (thermostating applies only in the pre-shock 300 K equilibration stages).

Force-field training

N/A — the manuscript uses a documented ReaxFF merge for RDX rather than introducing a wholly new parameterization workflow here.

Static QM / DFT

N/A — not the primary modality; QM references appear as literature context for chemistry benchmarks.

Findings

Shock focusing at a nanoscale void drives exothermic RDX decomposition fast enough that the hotspot does not quench within the first few ps of collapse. The largest pore develops a thin reaction zone propagating about ~0.25 km s⁻¹ with sharp temperature and composition fronts (abstract-level timescale ~10¹ ps). A statically heated control at comparable thermodynamic conditions does not reproduce the same deflagration-like phenomenology, so mechanical loading and momentum transport matter in this model beyond pure heating. The discussion contrasts this pathway with Tarver-style kinetics assumptions (J. Phys. Chem. C). Outcomes depend on pore diameter, crystal orientation, and particle velocity (Section 2.1). This repo’s Just Accepted PDF may differ from the VOR; use the on-disk PDF for exact tables.

Limitations

This repository copy is a Just Accepted manuscript PDF and may differ from the final version of record after production edits. The study isolates a single cylindrical void in an otherwise perfect crystal, so porosity statistics, defects, and 3D microstructure of real charges are not captured.

Relevance to group

Landmark ReaxFF + shock example for energetic materials, with explicit LANL/Purdue lineage tied to RDX chemistry and hot-spot physics.

Reader notes (navigation)

LANL/Strachan-line energetic materials corpus pairs naturally with RDX detonation and hot-spot physics entries elsewhere in the wiki. Prefer the VOR PDF for final tables and timesteps.