Renormalization of Molecular Quasiparticle Levels at Metal-Molecule Interfaces: Trends Across Binding Regimes

TL;DR

This paper investigates how substrate electrons screen added charges in adsorbed molecules, affecting quasiparticle energy levels, by comparing GW, Hartree-Fock, and Kohn-Sham calculations to identify key polarization mechanisms.

Contribution

It introduces a simple model to analyze universal trends in quasiparticle level renormalization at metal-molecule interfaces, highlighting two polarization mechanisms.

Findings

Metal polarization (image charge) dominates at high metal density of states.

Molecular polarization via charge transfer increases with metal-molecule coupling.

GW energies differ systematically from Hartree-Fock and Kohn-Sham results.

Abstract

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.