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

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 Ru 2+ -Ru 3+ 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. The smaller value for reorganization free energy can be explained by the 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.

Topological analyses of time-dependent electronic structures: application to electron-transfers in methionine enkephalin

“…In the present work we have used the Hirshfeld partitioning scheme [63]. Past studies indicate that the Hirshfeld scheme is suitable for dealing with ET using cDFT [60,64,65]. In deMon2k, the resolution of the KS equations is carried out within the framework of auxiliary DFT, meaning that auxiliary electronic density functions are used to calculate the Coulomb and the exchange-correlation potentials [66].…”

Transient Changes in Molecular Geometries and How to Model Them

“…Our method was able to qualitatively reproduce the Co-N bond elongation, and the cause for the quantitative discrepancy could be explained by the technical necessity of having to apply a finite FermiDirac distribution of the occupation of orbitals close to the HOMO-LUMO gap, in order to reach SCF convergence. A future correction of this problem could involve constrained DFT, a method which employs an external potential to further constrain the resulting electronic distribution, which has been used successfully before [192,193].…”

Progress and challenges in simulating and understanding electron transfer in proteins

“…(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) self-exchange reaction between Ru(II) and Ru(III) [85]. cDFT was also recently used to model ET between flavin cofactors in the protein emoB [86].…”