The interaction between molecules and inorganic surfaces plays an important role in organic electronics and catalysis. The predictive power of density functional theory (DFT) is instrumental in gaining a fundamental understanding of bonding characteristics and in characterizing and designing hybrid organic-inorganic interfaces. The simulation of core-level spectroscopies such as x-ray photoemission spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS), can form an important link between connecting atomistic structure and stability predictions from DFT with comparable experimental spectroscopic signatures. For this link to function, spectroscopic predictions should only be affected by controlled and systematic errors. In this talk, we will show that specific intermediate interaction regimes at molecule-metal interfaces can lead to compounding errors in XPS simulations that cause qualitatively wrong spectroscopic predictions. The well-characterized system of azulene adsorbed at Ag(111) and Cu(111) serves as our example case. Whereas constrained core-hole simulation methods such as the Delta-Self-Consistent-Field (ΔSCF) method for XPS simulations, and the ionization potential corrected transition potential (ΔIP-TP) method for NEXAFS simulations provide accurate predictions for the strongly bound azulene on Cu(111), the same is not the case for Ag(111). In this example, the presence of the core-hole can lead to spurious charge transfer and dynamic overscreening of core-level shifts. We find this effect to be closely connected to the intrinsic self-interaction error of approximate semi-local DFT functionals. This is evidenced by the use of a specific molecular-orbital-based self-interaction correction, which allows us to achieve much better agreement between DFT and experiment. We conclude by identifying the scenarios in which self-interaction-induced charge transfer artifacts can affect spectroscopic predictions at metal-organic interfaces.

Spurious charge transfer artefacts in core-level constrained x-ray spectroscopy simulations of metal-organic interfaces

ACS Spring 2022

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