Effects of Roaming Trajectories on the Transition State Theory Rates of a Reduced-Dimensional Model of Ketene Isomerization

The rates of chemical reactions (or any activated process) are by definition determined by the flux of reactants (or initial states) that end up as products (or final states). The forward flux through any surface that divides reactants from products is a constant as long as only those trajectories that were reactants in the infinite past and products in the infinite future are included in the flux once and only once. Transition state theory (TST) ignores this last clause, thereby overestimating the rate if any of the trajectories recross the dividing surface. However, its advantage is that it replaces a dynamical calculation with a statistical integral over the TST geometry. The recent identification of roaming trajectories—those that persist for a long time as neither reactant nor product without ever visiting near the col on the energy landscape—apparently challenges the dogma that TST’s only error lies in the omission of recrossing trajectories. This question is investigated using the isomerization rea...