Identifying Collisions of Various Molecularities in Molecular Dynamics Simulations

We present a method based on kinetic molecular theory that identifies reactions of various molecularities in molecular dynamics (MD) simulations of bulk gases. The method allows characterization of the thermodynamic conditions at which higher than bimolecular reactions are a factor in the mechanisms of complex gas-phase chemistry. Starting with Bodenstein's definition of termolecular collisions we derive analytical expressions for the frequency of higher molecularity collisions. We have developed a relationship for the ratio of the frequencies of termolecular to bimolecular collisions in terms of the temperature, density, and collision times. To demonstrate the method, we used ReaxFF in LAMMPS to carry out MD simulations for NVT ensembles of mixtures of H₂-O₂ over the density range 120.2-332.7 kg m^-3 and temperature range 3000-5000 K. The simulations yield ReaxFF-based predictions of the relative importance of termolecular collisions O₂···H₂···O₂ and bimolecular collisions O₂···H₂ in the early chemistry of hydrogen combustion.