Computing activation energies of non-thermal reactions

The chemistry in bulk gases involves reactions of nascent radicals that are almost invariably non-thermal. The energy requirements of reactions involving radicals depend on the reactions that produce them and the intra- and inter-molecular energy transfer they may undergo. Here, we extend the generalised Tolman activation energy (GTEa) method to non-thermal reactions in molecular dynamics (MD) simulations. We compute the energy requirements, which we refer to as chemical-activation energies (CE _a), of reactions of radicals formed by the decomposition of hydrogen peroxide. The equipartition theorem is adapted to compute average energies of small isolated systems with internal degrees of freedom in MD simulations with periodic boundary conditions, which is necessary for application of the GTEa method to non-thermal reactions. To illustrate the applicability of the GTEa method to non-thermal reactions, we present CE _a results for H₂O₂?+?OH → H₂O?+?HO₂, a key reaction in hydrogen combustion, as described by the ReaxFF force field. The OH radicals are the products of the self-dissociation of H₂O₂ and subsequent reactions. We define the chemical-activation energy for a back reaction (BCE _a) as the difference between the energy of the products and the average energy of the system. We show that the BCE _a and CEa are linearly correlated.