(584e) Analyzing the Performance of Uncoupled Mode Approximations in Reaction Rate Calculations

The harmonic oscillator model is often used to calculate reaction rate parameters such as the activation energy and the pre-exponential factor of the Arrhenius equation. However, because it fails to describe anharmonic modes such as torsions and intermolecular motions, the accuracy of the harmonic oscillator model is not always satisfactory. In this work, we explored the possibility of capturing anharmonic effects using the uncoupled mode (UM) approximation, where the full-dimensional potential energy surface (PES) is described as a sum of one-dimensional potentials of each mode. We calculated the kinetic properties of 19 chosen gas-phase reactions using the UM approximation to benchmark the performance of different PES sampling schemes. Compared to the harmonic oscillator model, the accuracy is not significantly improved when the one-dimensional potentials of UM are constructed by sampling along the direction of each normal mode (UM-N). However, sampling vibrational and torsional modes separately (UM-VT) leads to significant improvements, similar to the previously reported results for molecular thermodynamics computations. Furthermore, we compared the performance of the UM model in different coordinate systems, including redundant internal coordinates (RICs), hybrid internal coordinates (HICs), and translation-rotation-internal coordinates (TRICs). We found that HIC and TRIC better describe intermolecular motions in a transition state than RIC, because HIC adds the Cartesian coordinates of every atom to the primitive internal coordinates, and TRIC incorporates three translational and three rotational coordinates between each molecular fragment to model intermolecular movements. Among all the methods we examined, the RMS errors of the activation energy and pre-exponential factor in logarithm (log A) calculated using UM-VT with TRIC are the lowest, which are 1.17 (kcal/mol) and 0.68 (-), respectively.