General application of Tolman's concept of activation energy.

We present a generalization of Tolman's concept of activation energy applicable to thermal and non-thermal reactions in molecular dynamics simulations of reactions in bulk gases. To illustrate the applicability of the method, molecular dynamics calculations were carried out for the NVT ensemble to determine the activation energies of O2 + H2 → H + HO2 and 2O2 + H2 → 2HO2 from MD simulation results for [H2]/[O2] = 1 at 3000 K using the reactive force field, ReaxFF. Assuming local thermodynamic equilibrium, we define the reaction cluster local energy, the energy of the atoms participating in an individual reaction, which is conserved. The generalized Tolman activation energy (GTEa) approach is applicable to reactions of any molecularity. Although we have applied GTEa for thermal conditions, it is applicable to chemistry occurring under non-thermal conditions because it rests upon local rather than global equilibrium. We have defined the transition configurations, unique points that define a seam separating reactants and products at which the local energies of the reactants and products become equal.