ReaxFF

ReaxFF, short for reactive force field, is a family of empirical potential models designed for molecular dynamics simulations in which chemical bonds can form and break during the course of a simulation. Developed in the early 2000s by researchers led by Adnan R. van Duin and William A. Goddard III, ReaxFF uses a bond-order based formalism to represent chemistry across a wide range of elements and environments. It aims to provide a transferable description of reactive processes without explicitly specifying every possible reaction pathway.

The core feature of ReaxFF is dynamic bonding. Bond orders are computed from interatomic distances and influence a network of energy terms, including bond, angle, and torsion contributions, plus nonbonded interactions. Partial charges are assigned through a charge equilibration method (QEq or related schemes), allowing charges to evolve with the local environment to support reactions. The force field is parameterized for specific element combinations, enabling simulations of combustion, hydrocarbon chemistry, polymers, ceramics, catalysts, and energetic materials.

Parameter sets, often large, are the main limitation and must be developed or adapted for the system of interest. ReaxFF is implemented in several molecular dynamics codes, notably LAMMPS, and is available for numerous element combinations (C, H, O, N, S, P, halogens, and some metals). While more computationally intensive than fixed-charge force fields, ReaxFF enables explicit treatment of chemical reactivity in large-scale simulations, bridging quantum chemical accuracy and classical MD scale.