Ensemble-Based Molecular Simulation of Chemical Reactions under Vibrational Nonequilibrium

We present an approach to incorporate the effect of vibrational
nonequilibrium in molecular dynamics (MD) simulations. A perturbed
canonical ensemble, in which selected modes are excited to higher
temperature while all others remain equilibrated at low temperature,
is simulated by applying a specifically tailored bias potential. Our
method can be readily applied to any (classical or quantum mechanical)
MD setup at virtually no additional computational cost and allows
the study of reactions of vibrationally excited molecules in nonequilibrium
environments such as plasmas. In combination with enhanced sampling
methods, the vibrational efficacy and mode selectivity of vibrationally
stimulated reactions can then be quantified in terms of chemically
relevant observables, such as reaction rates and apparent free energy
barriers. We first validate our method for the prototypical hydrogen
exchange reaction and then show how it can capture the effect of vibrational
excitation on a symmetric SN2 reaction and radical addition
on CO2.