Length Dependence of Conductance in Aromatic Single-Molecule Junctions

Using a scattering-state approach incorporating self-energy corrections to the junction level alignment, the conductance G of oligophenyldiamine-Au junctions is calculated and elucidated. In agreement with experiment, we find G decays exponentially with the number of phenyls with decay constant beta = 1.7. A straightforward, parameter-free self-energy correction, including electronic exchange and correlations beyond density functional theory (DFT), is found to be essential for understanding the measured values of both G and beta. Importantly, our results confirm quantitatively the picture of off-resonant tunneling in these systems and show that exchange and correlation effects absent from standard DFT calculations contribute significantly to beta.