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 Schrödinger 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.