Correcting Rate Constants from Anharmonic Molecular Dynamics for Quantum Effects

Anharmonicity can
greatly affect rate constants. One or even several
orders of magnitude of deviation are found for obtaining rate constants
using the standard rigid-rotor harmonic-oscillator model. In turn,
reactive molecular dynamics (MD) simulations are a powerful way to
explore chemical reaction networks and calculate rate constants from
the fully anharmonic potential energy surface. However, the classical
nature of the dynamics and the required numerical efficiency of the
force field limit the accuracy of the resulting kinetics. We combine
the best of both worlds by presenting an approximation that pairs
anharmonic information intrinsic to classical MD with high-accuracy
energies and frequencies from quantum-mechanical electronic structure
calculations. The proposed scheme is applied to hydrogen abstractions
in the methane system, which allows for the benchmarking of rate constants
corrected by our approach against experimental rate constants. This
comparison reveals a standard deviation of factor 2.6. Two archetypes
of possible failure are identified in the course of a detailed investigation
of the CH3• + H• → CH22• + H2 reaction. From this
follows the application range of the method, within which the method
shows a standard deviation of factor 2.1. The computational efficiency
and beneficial scaling of the method allow for application to larger
systems, as shown for hydrogen abstraction from 2-butanone by HO2•.