Skip to content

Using C-DFT to develop an e-ReaxFF force field for acetophenone radical anion

Scope

e-ReaxFF extension for the acetophenone radical anion using constrained DFT (C-DFT) targets on each atomic center, building on the 2021 Akbarian e-ReaxFF parameterization for cross-linked polyethylene (XLPE) chemistry.

Summary

Cross-linking polyethylene with dicumyl peroxide yields XLPE for high-voltage cable insulation but also by-products such as acetophenone, which has favorable electron affinity. The paper develops an e-ReaxFF description of the acetophenone radical anion for use in reactive molecular dynamics of degradation chemistry. Starting from the existing XLPE-oriented e-ReaxFF parameter set, the authors run C-DFT geometry optimizations with the excess electron constrained separately to each atom of acetophenone, add the resulting energies to the e-ReaxFF training set, and iteratively refit until e-ReaxFF reproduces the C-DFT data. They compare equilibrium populations from energy-minimized structures between C-DFT and e-ReaxFF across temperatures, and they test MD by comparing e-ReaxFF electronic distributions to unconstrained-DFT spin densities.

Methods

Static QM (C-DFT for eReaxFF training). Constrained DFT (C-DFT) geometry optimizations place the excess electron of the acetophenone radical anion on each heavy-atom center in turn, yielding site-resolved energies that standard unconstrained DFT would over-delocalize. The full functional, dispersion, and basis specifications are in J. Chem. Phys. 155, 214104.

Force-field training (eReaxFF extension). The eReaxFF description extends the 2021 Akbarian eReaxFF parameterization used for XLPE-related chemistry. Constrained-DFT total energies and geometries enter the eReaxFF training set; the parameters are reoptimized until RMS agreement is acceptable (least-squares / Monte Carlo updates as described for eReaxFF in the paper). Unconstrained DFT VASP spin densities provide an independent check on charge/spin distributions; covalent chemistry outside the acetophenone fragment is not refit here.

MD application (eReaxFF in LAMMPS). eReaxFF molecular dynamics in LAMMPS compares trajectory-averaged on-site electronic populations to unconstrained DFT spin density maps for the radical anion. The letter reports NVT sampling with a Nosé–Hoover-type thermostat; time step, temperature list, and equilibration vs production durations are in Section 2 of the JCP issue and the SI. Barostat — N/A in the highlighted NVT validation blocks. Hydrostatic pressure (bar) — N/A as an independent MD control in these NVT demonstrations (fixed cell volume for the stated tests). Static external electric field in MD — N/A. PBC and box details for the acetophenone test cells: see the article and SI. Shock, shear, metadynamics — N/A for the stated eReaxFF tests.

Findings

Fitting and thermodynamics. The refitted eReaxFF set reproduces the C-DFT site energies well enough to close the training loop, and the paper reports multi-temperature mixture behavior consistent with the same C-DFT references (see tables and discussion in the VOR).

MD validation. Trajectory-averaged eReaxFF on-site electronic weights qualitatively track unconstrained DFT spin-density features for the radical anion, supporting the intended use in larger ReaxFF+eReaxFF insulation-chemistry models.

Sensitivity and scope. The study varies temperature in the eReaxFF tests it reports; it does not benchmark cable aging in a laboratory under high field. The JCP text limits claims to the acetophenone radical anion in the XLPE-aligned chemistry space—not a full cable-additive or electrolyte eReaxFF fit (see ## Limitations and the article Discussion).

Limitations

The study focuses on acetophenone and its radical anion in the stated electronic-structure approximations; broader electrolyte or cable additives are outside the demonstrated training set. Corpus note: long XLPE insulation field biases and real electrolyte solutions are not parameterized by this C-DFT+eReaxFF fragment alone; see pdf_path Discussion for outlook.

Relevance to group

Extends e-ReaxFF for high-voltage polymer insulation chemistry with explicit charge-localization training via C-DFT.

Citations and evidence anchors