Reactive simulations-based model for the chemistry behind condensed phase ignition in RDX crystals from hot spots
Note on the PDF
The corpus PDF is a Royal Society of Chemistry Accepted Manuscript (pre-technical editing). Scientific claims below follow this file and the abstract text.
Summary¶
Reactive molecular dynamics with ReaxFF is used to study condensed-phase ignition of high-pressure γ-RDX after localized thermal hot spots are created by heating a small fraction of the crystal. After the pulse, dynamics continue under adiabatic (NVE) integration so incubation and runaway reflect exothermic chemistry rather than fixed-temperature constraints. The paper’s central claim is that condensed-phase pathways differ qualitatively from common gas-phase unimolecular pictures for nitramines: hydrogen-transfer chemistry and polyradical intermediates matter before NO₂-first mechanisms dominate.
Methods¶
- Crystal phase and cell: γ-RDX at experimental density ~2.26 g cm⁻³; unit cell from CASTEP PBE DFT-D relaxations, then ReaxFF relaxation; supercell 4×4×4 (512 RDX molecules, ~50.24×37.92×43.68 Å).
- Hot-spot protocol: Central region (~5% by volume, 24 molecules) receives a thermal pulse at 1000 K or 2000 K for 5–40 ps; during the pulse the hot zone uses NVT (1000 K or 2000 K) while the remainder is NVE; after the pulse the entire system is NVE. Initial equilibration 300 K, 100 000 steps, Δt = 0.01 fs; production Δt = 0.01 fs throughout.
- Force field / code: ReaxFF in LAMMPS, combining the Wood et al. nitramine CHNO set with C/H/O combustion training (Chenoweth et al.); original ReaxFF form (not ReaxFF-lg) as stated for dense γ-RDX combustion chemistry.
- PBC / ensembles: 3D periodic γ-RDX supercell. During the pulse, the hot zone is held at 1000 K or 2000 K with NVT while the remainder is NVE; after the pulse the entire system runs NVE at fixed volume. The NVT subregion implements thermostat-like temperature control as described in the PCCP Methods for their LAMMPS setup. Barostat / applied pressure: N/A — not used in this fixed-cell hot-spot protocol.
Findings¶
Longer thermal pulses on the central hot zone favor an incubation period followed by thermal runaway; shorter pulses let the cell re-equilibrate without ignition within the simulated window. During incubation, inter- and intramolecular hydrogen transfer dominates, while N–N scission to NO\(_2\)—typical of gas-phase unimolecular nitramine pictures—is suppressed in this dense γ-RDX setup. Polyradicals retaining triazine rings accumulate before ring-opening chemistry accelerates exothermic release of N\(_2\), H\(_2\)O, and CO\(_2\). The authors stress finite system size, picosecond–nanosecond horizons, and the idealized hot-spot protocol versus shock or laser energy deposition. This note tracks the Just Accepted PDF in pdf_path; use the issue-of-record PCCP PDF for stable figure numbering.
Limitations¶
Accepted-manuscript formatting; finite system size and short timescales; hot-spot protocol is a stylized stand-in for laser/shock initiation. Validation against higher-level electronic structure is described only at selected points in the paper. The γ-RDX crystal setup and adiabatic continuation are idealizations relative to real defects, voids, and multi-crystal textures in pressed formulations.
Relevance to group¶
Illustrates ReaxFF applied to nitramine condensed-phase ignition chemistry and hot-spot–to–runaway sequencing. Complements other energetic materials reactive MD notes in the KB that emphasize gas-phase kinetics instead of condensed hydrogen-transfer sequences.
Citations and evidence anchors¶
DOI 10.1039/C5CP00950B.