•M. Koentopp, F. Weigend und F. Evers
Institut für Nanotechnologie, Forschungszentrum Karlsruhe, 76128 Karlsruhe, Germany

We report calculations using density functional theory (DFT) for the conductance of an organic molecule that has been studied experimentally by J. Reichert et al. (PRL 88, 176804 (2002)). Our calculations are based on the nonequilibrium Green's function formalism and on density functional calculations using TURBOMOLE. Each gold electrode has been simulated by 55 Au-atoms that take the form of a pyramid. This number is large enough to ensure that Fermi level mismatch does not occur. Moreover the charge transfer from the electrodes to the molecule affects a spatial region with an extension small compared to the total volume of the pyramid. Therefore charging effects are taken into account in an essentially exact manner. We find good qualitative agreement with experimental results. In order to address possible reasons for quantitative discrepancies a detailed study of the conductance change upon modification of the microscopic conditions is presented. Also, we compare results from the DFT calculations to other calculations based upon the Hartree-Fock and the extended Hückel method. This analysis suggests, that quantitative discrepancies are at least in part due to the self interaction problem inherent in most commonly used DFT functionals and probably not primarily due to the details of the microstructure realized in the experiment.