Bimolecular reaction rates from ring polymer molecular dynamics.

Area of Science:

• Chemical Kinetics
• Computational Chemistry
• Quantum Dynamics

Background:

• Accurate calculation of bimolecular reaction rates is crucial in chemical kinetics.
• Classical methods often fail to capture quantum mechanical effects like tunneling at low temperatures.
• The ring polymer molecular dynamics (RPMD) approximation offers a potential route to include these effects.

Purpose of the Study:

• To develop and validate an efficient RPMD-based procedure for calculating gas-phase bimolecular reaction rates.
• To assess the accuracy of the RPMD method compared to exact quantum scattering calculations across different temperature regimes.
• To explore the applicability of RPMD to complex, full-dimensionality chemical reactions.

Main Methods:

• A novel RPMD procedure for calculating reaction rates without requiring absolute partition functions.
• Utilizing thermodynamic integration to obtain the ratio of partition functions for reactants and transition states.
• Applying the method to benchmark 3D reactions: H + H2, Cl + HCl, and F + H2.

Main Results:

• The RPMD method efficiently calculates reaction rates without needing absolute partition functions.
• RPMD rate coefficients show excellent agreement with exact quantum results at high temperatures.
• At low temperatures, RPMD results are within a factor of 3 of exact values, significantly outperforming classical methods in the deep quantum tunneling regime.

Conclusions:

• The developed RPMD procedure is an effective and efficient method for calculating gas-phase bimolecular reaction rates.
• The method shows high accuracy, particularly in quantum tunneling regimes where classical approximations fail.
• The approach holds promise for future applications to more complex chemical reactions in full dimensionality, with potential for low-temperature improvements.