Partial vs. integer electron transfer in molecular assemblies: On the importance of a multideterminant theoretical description and the necessity to find a solution within DFT

TL;DR

This paper evaluates the limitations of standard DFT-based NEGF methods in describing electron transfer in molecular junctions, especially in the Coulomb blockade regime, and proposes a practical scheme to improve the extraction of addition energies.

Contribution

It demonstrates the failure of mean-field DFT approaches in weakly coupled systems and introduces a new scheme to accurately determine addition energies in the Coulomb blockade regime.

Findings

Standard NEGF-DFT methods fail in Coulomb blockade regime.

Proposed scheme accurately extracts addition energies from NEGF-DFT.

Electrostatic screening effects are incorporated in the simple approach.

Abstract

Nonequilibrium Greens function techniques (NEGF) combined with density functional theory (DFT) calculations have become a standard tool for the description of electron transport through single molecule nanojunctions in the coherent tunneling regime. However, the applicability of these methods for transport in the Coulomb blockade regime is questionable. For a molecular assembly model, with multideterminant calculations as a benchmark, we show how a closed shell ansatz, the usual ingredient of meanfield methods, fails to properly describe the step like electron transfer characteristic in weakly coupled systems. Detailed analysis of this misbehavior allows us to propose a practical scheme to extract the addition energies in the CB regime for single-molecule junctions from NEGF DFT within the local density approximation (closed shell). We show also that electrostatic screening effects are taken into account within this simple approach.