By Kristian S. Thygesen and Angel Rubio from Phys. Rev. Lett. 102, 046802 (2009)
The position of an adsorbed molecule's frontier orbitals with respect to the substrate Fermi level determines the threshold energies at which electron transfer can take place across the metal-molecule interface. Such electron transfer processes represent a cornerstone of surface science and form the basis of photo- and non-adiabatic chemistry, organic- and molecular electronics, as well as scanning tunneling- and photoemission spectroscopy [1-5]. Accurate descriptions of adsorbate energy spectra are thus fundamental for quantitative modeling within these important areas.
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.
(a) Schematic of a molecule's HOMO and LUMO energy levels as it approaches a metal surface. For weak coupling (physisorbed molecule) the gap is reduced due to image charge formation in the metal. For strong coupling (chemisorbed molecule) dynamic charge transfer between molecule and metal reduces the gap further.
(b) The model used in the present study.
(c) The semi-elliptical band at the terminal site of the TB chain and the resonances of the molecule.
 J. W. Gadzuk, Phys. Rev. Lett. 76, 4234 (1996)
 Cuniberti, G., Fagas, G., and Richter, K., Introducing molecular electronics, Springer 2005.
 A. Nitzan and M. A. Ratner, Science bf 300, 1384 (2003)
 J. Repp et al. Phys. Rev. Lett. 94, 026803 (2005)
 X. Lu et al. Phys. Rev. B 70, 115418 (2004)