Rate constants from instanton theory via a microcanonical approach

TL;DR

This paper introduces three new algorithms to compute stability parameters in microcanonical instanton theory, enabling more stable and accurate rate constant calculations for quantum tunneling in chemical reactions across various systems.

Contribution

It presents alternative algorithms for calculating stability parameters in non-separable systems within microcanonical instanton theory, improving numerical stability.

Findings

Algorithms successfully applied to molecular systems with different potential energy surfaces.

Enhanced numerical stability in calculating rate constants for quantum tunneling.

Demonstrated applicability to reactions involving complex vibrational couplings.

Abstract

Microcanonical instanton theory offers the promise of providing rate constants for chemical reactions including quantum tunneling of atoms over the whole temperature range. We discuss different rate expressions, which require the calculation of stability parameters of the instantons. The traditional way of obtaining these stability parameters is shown to be numerically unstable in practical applications. We provide three alternative algorithms to obtain such stability parameters for non-separable systems, i.e., systems in which the vibrational modes perpendicular to the instanton path couple to movement along the path. We show the applicability of our algorithms on two molecular systems: H$_2$ + OH $\rightarrow$ H$_2$O + H using a fitted potential energy surface and HNCO + H $\rightarrow$ NH$_2$CO using a potential obtained on-the-fly from density functional calculations.