Multideterminant Assessment of Mean Field Methods for the Description of Electron Transport in the Weak Coupling Regime

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 steplike 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 behavior can be obtained with a spin-unrestricted (i.e., spin-polarized) ansatz although 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.