Instantons describe the tunneling pathways that dominate quantum dynamics at low temperatures. Moreover, they provide a link between the classical and quantum mechanical descriptions of chemical kinetics in what is called instanton rate theory. Derived from Feynman's path integral interpretation of quantum mechanics, this semiclassical theory allows for efficient computation of reaction rates in systems where the full quantum description would be too demanding. It is able to include quantum effects such as tunneling and zero-point energy while keeping the resourcefulness of a classical rate calculation. So far instanton rate theory has almost exclusively been used to calculate thermal rate constants k(T) on adiabatic potential energy surfaces where quantum effects have a large effect. Recent advancements have allowed us to calculate deep-tunneling rates even more efficiently and with favorable scaling. Unimolecular hydrogen transfer rates for the CH3CHOO Criegee intermediate were calculated to within a factor of 2 of experimental results using an on-the-fly surface. Bimolecular rates for the H+CH4 reaction were also calculated to within this factor using a fitted surface. We also present a new microcanonical formulation of instanton theory which allows for calculation of accurate energy-dependent rates k(E). This in turn will be very useful to atmospheric chemists who are interested in quantifying energy-resolved rates for molecular reactions. Microcanonical instantons can introduce quantum effects into statistical theories like RRKM with only a slight increase in computational cost. Furthermore, this approach works in full dimensionality and requires no parametrization or ad hoc corrections.

Calculation of reaction rates in the atmosphere using instanton theory: Rigorous inclusion of tunneling and zero-point energy

2019 Fall National Meeting

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