  On the basis of first-principles G0W0 calculations we study systematically
how the electronic levels of a benzene molecule are renormalized by substrate
polarization when physisorbed on different metallic and semiconducting
surfaces. The polarization-induced reduction of the energy gap between occupied
and unoccupied molecular levels is found to scale with the substrate density of
states at the Fermi level (for metals) and substrate band gap (for
semiconductors). These conclusions are further supported by GW calculations on
simple lattice models. By expressing the electron self-energy in terms of the
substrate's joint density of states we relate the level shift to the surface
electronic structure thus providing a microscopic explanation of the trends in
the G0W0 calculations. While image charge effects are not captured by
semi-local and hybrid exchange-correlation functionals, we find that error
cancellations lead to remarkably good agreement between the G0W0 and Kohn-Sham
energies for the occupied orbitals of the adsorbed molecule.
