On the calculation of quantum mechanical electron transfer rates

TL;DR

This paper introduces a simple interpolation formula for electron transfer rates that combines existing computational methods to accurately predict rates across different electronic coupling regimes, accounting for nuclear tunneling and solvent effects.

Contribution

The paper presents a novel interpolation formula that seamlessly integrates Fermi Golden Rule and Born-Oppenheimer rates, enhancing the accuracy of electron transfer rate calculations across all coupling strengths.

Findings

The formula accurately reproduces exact quantum rates in a 1D scattering problem.

It effectively captures nuclear tunneling, zero point energy, and solvent friction effects.

Performance is comparable to hierarchical equations of motion (HEOM) results.

Abstract

We present a simple interpolation formula for the rate of an electron transfer reaction as a function of the electronic coupling strength. The formula only requires the calculation of Fermi Golden Rule and Born-Oppenheimer rates and so can be combined with any methods that are able to calculate these rates. We first demonstrate the accuracy of the formula by applying it to a one dimensional scattering problem for which the exact quantum mechanical, Fermi Golden Rule, and Born-Oppenheimer rates are readily calculated. We then describe how the formula can be combined with the Wolynes theory approximation to the Golden Rule rate, and the ring polymer molecular dynamics (RPMD) approximation to the Born-Oppenheimer rate, and used to capture the effects of nuclear tunnelling, zero point energy, and solvent friction on condensed phase electron transfer reactions. Comparison with exact hierarchical equations of motion (HEOM) results for a demanding set of spin-boson models shows that the interpolation formula has an error comparable to that of RPMD rate theory in the adiabatic limit, and that of Wolynes theory in non-adiabatic limit, and is therefore as accurate as any method could possibly be that attempts to generalise these methods to arbitrary electronic coupling strengths.