Multistate ring polymer instantons and nonadiabatic reaction rates

TL;DR

This paper introduces two formulations of multistate ring polymer instantons for quantum reaction rate calculations, demonstrating their effectiveness in both adiabatic and nonadiabatic regimes through model system analysis.

Contribution

The paper develops and compares two novel RPI methods for multistate systems, enhancing the accuracy and robustness of quantum reaction rate computations.

Findings

MF-RPI agrees with previous results for symmetric systems

Numerical techniques are robust for systems with non-zero driving force

MF-RPI accurately computes rate constants across a wide range of coupling strengths

Abstract

We present two multistate ring polymer instanton (RPI) formulations, both obtained from an exact path integral representation of the quantum canonical partition function for multistate systems. The two RPIs differ in their treatment of the electronic degrees of freedom; whereas the Mean-Field (MF)-RPI averages over the electronic state contributions, the Mapping Variable (MV)-RPI employs explicit continuous Cartesian variables to represent the electronic states. We compute both RPIs for a series of model two-state systems coupled to a single nuclear mode with electronic coupling values chosen to describe dynamics in both adiabatic and nonadiabatic regimes. We show that the MF-RPI for symmetric systems are in good agreement with previous literature, and we show that our numerical techniques are robust for systems with non-zero driving force. The nuclear MF-RPI and the nuclear MV-RPI are similar, but the MV-RPI uniquely reports on the changes in the electronic state populations along the instanton path. In both cases, we analytically demonstrate the existence of a zero-mode and we numerically and that these solutions are true instantons with a single unstable mode as expected for a first order saddle point. Finally, we use the MF-RPI to accurately calculate rate constants for adiabatic and nonadiabatic model systems with the coupling strength varying over three orders of magnitude.