Correcting Rate Constants from Anharmonic Molecular Dynamics for Quantum Effects

Abstract

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• → CH2 2• + 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 •.

Coupled Anharmonic Thermochemistry from Stratified Monte-Carlo Integration

“…Situations where this does not hold true can produce gross inaccuracies in the approximation. 63 Methods CIMCI is a semi-classical method for obtaining anharmonic partition functions developed in this study. It is similar in its overall approach to MCPSI, but aims to address some of its shortcomings.…”

Coupled Anharmonic Thermochemistry from Stratified Monte-Carlo Integration

“…Situations where this does not hold true can produce gross inaccuracies in the approximation. 63 CIMCI is a semi-classical method for obtaining anharmonic partition functions developed in this study. It is similar in its overall approach to MCPSI, but aims to address some of its shortcomings.…”

The hindered rotor theory: A review

“…18,25 During a hindered rotation, one molecular fragment (called "the top or hindered rotor, HR") rotates relative to another (called "the frame") around a bond (Figure 1). 12,18,26 Two crucial input parameters for the treatment of the hindered rotor arise from these definitions: the torsional barrier and frequency. Other parameters are the torsional symmetry number and the reduced moment of inertia (vide infra).…”