  When an electron or a hole is added into an orbital of an adsorbed molecule
the substrate electrons will rearrange in order to screen the added charge.
This results in a reduction of the electron addition/removal energies as
compared to the free molecule case. In this work we use a simple model to
illustrate the universal trends of this renormalization mechanism as a function
of the microscopic key parameters. Insight of both fundamental and practical
importance is obtained by comparing GW quasiparticle energies with Hartree-Fock
and Kohn-Sham calculations. We identify two different polarization mechanisms:
(i) polarization of the metal (image charge formation) and (ii) polarization of
the molecule via charge transfer across the interface. The importance of (i)
and (ii) is found to increase with the metal density of states at the Fermi
level and metal-molecule coupling strength, respectively.
