Rate constants from instanton theory via a microcanonical approach

Area of Science:

• Chemical Physics
• Quantum Chemistry
• Computational Chemistry

Background:

• Microcanonical instanton theory aims to compute chemical reaction rate constants across all temperatures.
• Calculating stability parameters for instantons is crucial for accurate rate expressions.
• Traditional methods for obtaining these parameters are numerically unstable for practical applications.

Purpose of the Study:

• To address the numerical instability of traditional methods for calculating instanton stability parameters.
• To develop and present three alternative, numerically stable algorithms for obtaining stability parameters.
• To demonstrate the applicability of these new algorithms on relevant molecular systems.

Main Methods:

• Development of three novel algorithms for calculating instanton stability parameters.
• Application of these algorithms to non-separable systems where perpendicular vibrational modes couple to the reaction path.
• Utilizing density functional calculations for on-the-fly potential energy surface generation.

Main Results:

• The proposed algorithms provide numerically stable methods for obtaining stability parameters.
• Successful application of the algorithms to the H₂ + OH reaction and the HNCO + H reaction.
• Demonstrated the effectiveness of the methods for both fitted and on-the-fly calculated potential energy surfaces.

Conclusions:

• The new algorithms offer a robust and stable approach to calculating essential parameters in microcanonical instanton theory.
• These advancements facilitate more accurate predictions of chemical reaction rates, particularly for complex systems.
• The study validates the utility of the developed methods in practical chemical dynamics simulations.