Molecular Conductance from Ab Initio Calculations: Self Energies and Absorbing Boundary Conditions

Calculating an exact self energy for ab initio transport calculations relevant to ``Molecular Electronics'' can be troublesome. Errors or insufficient approximations made at this step are often the reason why many molecular transport studies become inconclusive. We propose a simple and efficient approximation scheme, that follows from interpreting the self energy as an absorbing boundary condition of an effective Schroedinger equation. In order to explain the basic idea, a broad introduction into the physics incorporated in these self energies is given. The method is further illustrated using a tight binding wire as a toy model. Finally, also more realistic applications for transport calculations based on the density functional theory are included.