Realistic, quantitative descriptions of electron-transfer reactions: diabatic free-energy surfaces from first-principles molecular dynamics

TL;DR

This paper introduces a first-principles molecular dynamics method to accurately compute diabatic free-energy surfaces for electron-transfer reactions, exemplified by ferrous-ferric ion exchange in water.

Contribution

It presents a novel approach combining density-functional theory with a penalty functional to calculate diabatic surfaces and correct self-interaction errors.

Findings

Accurate diabatic free-energy surfaces for electron transfer were obtained.

The method successfully modeled the ferrous-ferric ion exchange in water.

The approach improves the reliability of first-principles electron-transfer simulations.

Abstract

A general approach to calculate the diabatic surfaces for electron-transfer reactions is presented, based on first-principles molecular dynamics of the active centers and their surrounding medium. The excitation energy corresponding to the transfer of an electron at any given ionic configuration (the Marcus energy gap) is accurately assessed within ground-state density-functional theory via a novel penalty functional for oxidation-reduction reactions that appropriately acts on the electronic degrees of freedom alone. The self-interaction error intrinsic to common exchange-correlation functionals is also corrected by the same penalty functional. The diabatic free-energy surfaces are then constructed from umbrella sampling on large ensembles of configurations. As a paradigmatic case study, the self-exchange reaction between ferrous and ferric ions in water is studied in detail.