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

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.