Most recent theoretical studies of electron transport in single-molecule junctions rely on a Landauer approach, simplified to treat electron-electron interactions at a mean-field level within density functional theory (DFT). While this framework has proven relatively accurate for certain systems, such as metallic point contacts, the computed conductance often substantially exceeds the measured values for organic molecules. This disagreement has raised questions about the validity of static DFT, inherently a ground state theory, for computing electronic transport properties.
Fundamentally, charge transport through metal-molecule junctions is determined by the electronic coupling of frontier molecular orbitals to extended states in the metal, and their energetic position relative to the Fermi energy. However, frontier molecular levels correspond to electron removal (ionization) and addition (affinity) energies, neither of which can be well described by orbital energies computed with DFT.
Here I will discuss the use of an established, first-principles many-electron self-energy approach, within the GW approximation, to study the impact of correlation on electronic level alignment at physisorbed metal-organic interfaces. I will initially describe results for benzene on graphite, a prototype physisorbed metal-molecule contact for which surface polarization effects are found to drastically modify frontier orbital energies . From these results, a model correlation correction to static DFT resonance energies is developed for weakly-coupled molecular junctions, and then shown to reconcile the conductance of benzenediamine-Au junctions, where the average computed value  was found to be about seven times larger than experiment .
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- S. Y. Quek, L. Venkataraman, H. J. Choi, S. G. Louie, M. S. Hybertsen, and J. B. Neaton, submitted (2007).
- L. Venkataraman, J. E. Klare, C. Nuckolls, M. S. Hybertsen, and M. L. Steigerwald, Nature 442, 904 (2006).
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