  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.
