A New Reactive Force Field for NOx-Water Interactions: Development, Evaluation, and Molecular Dynamics Simulation of...

Reactive nitrogen species (RNSs) are key contributors to plasma-liquid processes and play a critical role in the chemistry of plasma-activated water (PAW). Despite their importance, their molecular-level behavior in aqueous environments is still not well understood, largely due to the absence of reliable reactive force fields. In this study, a new ReaxFF parameter set, ReaxFFCHONx-2026, is developed and systematically assessed for modeling nitrogen- and oxygen-containing reactive species under both gas-phase and aqueous conditions relevant to plasma systems. The parameter set is optimized using density functional theory (DFT) reference data for representative NOx reaction pathways and integrated with an existing aqueous/biomolecular ReaxFF framework to enable reactive molecular dynamics simulations in water. Its performance is validated through a range of structural, thermodynamic, and dynamic properties including radial distribution functions, potential energy profiles, and diffusion coefficients. The results demonstrate strong agreement with DFT calculations and previously reported data, particularly in describing N-O bond breaking and solvation behavior. As a case study, the interaction of peroxynitrite (ONOO-) with water is investigated using reactive molecular dynamics, where atomic charges are dynamically determined via the ReaxFF charge equilibration method. The simulations show protonation leading to ONOOH formation, pronounced site-specific solvation around oxygen atoms, and preferential cleavage of the peroxide O-O bond. Analysis of diffusion further indicates that the predicted transport properties of RNS are consistent with the available data, supporting the reliability of the developed force field. Overall, ReaxFFCHONx-2026 offers a robust framework for large-scale atomistic simulations of PAW and reactive nitrogen chemistry.