  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.
