Charge constrained density functional molecular dynamics for simulation of condensed phase electron transfer reactions

TL;DR

This paper introduces a plane-wave based charge constrained density functional molecular dynamics method to simulate electron transfer reactions in condensed phases, applied to a ruthenium complex in aqueous solution, providing insights into reorganization energies.

Contribution

The paper develops and applies a plane-wave implementation of CDFT-MD for condensed phase electron transfer, including new expressions for constraint forces and analysis of charge definitions.

Findings

Reorganization free energy of 1.6 eV for Ru complex

Lower reorganization energy than continuum solvation models

Similar Ru-O distances in transfer complex reduce reorganization energy

Abstract

We present a plane-wave basis set implementation of charge constrained density functional molecular dynamics (CDFT-MD) for simulation of electron transfer reactions in condensed phase systems. Following earlier work of Wu et al. Phys. Rev. A 72, 024502 (2005), the density functional is minimized under the constraint that the charge difference between donor and acceptor is equal to a given value. The classical ion dynamics is propagated on the Born-Oppenheimer surface of the charge constrained state. We investigate the dependence of the constrained energy and of the energy gap on the definition of the charge, and present expressions for the constraint forces. The method is applied to the Ru2+-Ru3+ electron self-exchange reaction in aqueous solution. Sampling the vertical energy gap along CDFT-MD trajectories, and correcting for finite size effects, a reorganization free energy of 1.6 eV is obtained. This is 0.1-0.2 eV lower than a previous estimate based on a continuum model for solvation. smaller value for reorganization free energy can be explained by fact that the Ru-O distances of the divalent and trivalent Ru-hexahydrates are predicted to be more similar in the electron transfer complex than for the separated aqua-ions.