•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.