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Iodine recombination in xenon solvent: Clusters in the gas to liquid-like state transition

Abstract

A Xe/I ReaxFF parameter set is trained on DFT, CCSD, and CASSCF reference data, then used in ReaxFF MD of supercritical xenon with iodine to connect xenon cluster lifetimes and thermodynamic state to iodine geminate recombination; Berendsen NVT controls with 1 fs (pure Xe) or 0.25 fs (iodine recombination) timesteps are reported, with ten replica simulations for averaging.

Summary

The work targets iodine recombination in xenon across gas-like, liquid-like, and supercritical conditions, emphasizing solvent clustering and the cage effect. A Xe/I ReaxFF description is fit to quantum data (DFT surfaces for Xe–Xe, Xe–I, I₂ configurations; CCSD for Xe–Xe; CASSCF for I–I where DFT underbinds; Gaussian 16 and ADF/AMS for specific scans). Reactive MD then probes xenon cluster statistics and iodine recombination under NVT (and variants with multiple thermostats) versus brief NVE discussion.

Methods

Quantum training data

  • DFT: PBE with def2-TZVP and Grimme D2 dispersion for most dimers; CCSD (DGDZVP) for Xe–Xe; CASSCF(6,8) for I–I; additional PBE/TZP scans in ADF/AMS for I–I–I angles. Non-periodic cluster calculations.
  • ReaxFF optimization: Single-parameter search minimizing weighted squared error between QM energies and ReaxFF (Eq. (2) in the paper).

ReaxFF MD protocols

Integration uses LAMMPS as the molecular dynamics engine for the published protocol (see JCP Methods for verbatim commands).

  • Pure xenon validation: Energy-minimized boxes; sizes 500–10 000 atoms; states spanning liquid, supercritical, gas (~202.6–636.6 K, ~58.98–102 bar in the reported table). NVT with Berendsen thermostat (damping 1000 fs); 1 fs timestep; 1 ns duration for solvent characterization. 3D PBC cubic supercells for the Xe boxes in these validation runs.
  • Iodine recombination: I : Xe ≈ 1 : 40; NVT with Berendsen (1000 fs damping) or multi-thermostat NVT (Xe damping 1000 fs, I damping 1×10⁶ fs). 0.25 fs timestep; 1 ns per run; 10 independent replicas ensemble-averaged. N/A — NPT barostat in the production runs summarized (constant-volume NVT for these cells). N/A — external electric field; N/A — metadynamics (not the focus of the reactive MD protocol described here).

Findings

  • Clusters: Xe cluster lifetimes on the order of ~5–11 ps are comparable to iodine geminate recombination timescales in the analysis.
  • Recombination: Simulations report higher probability of I₂ formation when xenon clusters are present, and that supercritical conditions show the highest recombination rate among the states highlighted in the abstract and discussion. The AIP galley pdf_path should be cross-checked against the VOR JCP PDF for any page-level differences.

Limitations

The corpus PDF is an AIP author proof (query form, placeholder page numbers in the header). Software branding for the MD engine is not stated explicitly in the extracted text.

Relevance to group

A. C. T. van Duin is a co-author; the paper is ReaxFF parameterization for Xe/I reactive solvent chemistry.

Citations and evidence anchors

  • DOI: 10.1063/5.0260087 — J. Chem. Phys. (galley PDF in corpus).