A multideterminant assessment of mean field methods for the description of electron transfer in the weak coupling regime

TL;DR

This study evaluates the limitations of mean-field methods like Hartree Fock and DFT in accurately modeling electron transfer in weakly coupled systems, highlighting the importance of multideterminant approaches and spin-polarization effects.

Contribution

It demonstrates that standard mean-field methods fail to capture the step-like electron transfer behavior, and assesses the improvements from spin-unrestricted solutions compared to multideterminant benchmarks.

Findings

Closed-shell mean-field methods do not reproduce step-like electron transfer.

Spin-unrestricted Hartree Fock and DFT can qualitatively capture transfer steps.

Multideterminant calculations serve as benchmarks for assessing mean-field approximations.

Abstract

Multideterminant calculations have been performed on model systems to emphasize the role of many-body effects in the general description of charge quantization experiments. We show numerically and derive analytically that a closed-shell ansatz, the usual ingredient of mean-field methods, does not properly describe the step-like electron transfer characteristic in weakly coupled systems. With the multideterminant results as a benchmark, we have evaluated the performance of common ab initio mean field techniques, such as Hartree Fock (HF) and Density Functional Theory (DFT) with local and hybrid exchange correlation functionals, with a special focus on spin-polarization effects. For HF and hybrid DFT, a qualitatively correct open-shell solution with distinct steps in the electron transfer behaviour can be obtained with a spin-unrestricted (i.e., spin-polarized) ansatz though this solution differs quantitatively from the multideterminant reference. We also discuss the relationship between the electronic eigenvalue gap and the onset of charge transfer for both HF and DFT and relate our findings to recently proposed practical schemes for calculating the addition energies in the Coulomb blockade regime for single molecule junctions from closed-shell DFT within the local density approximation.