Final-State Simulations of Core-Level Binding Energies at Metal-Organic Hybrid Interfaces: Artifacts Caused by Spurious Collective Electrostatic Effects

Abstract

Core-level energies are frequently calculated to explain the X-ray photoelectron spectra of metal-organic hybrid interfaces. The current paper describes how such simulations can be flawed when modeling interfaces between physisorbed organic molecules and metals. The problem occurs when applying periodic boundary conditions to correctly describe extended interfaces and simultaneously considering core hole excitations in the framework of a final-state approach to account for screening effects. Since the core hole is generated in every unit cell, an artificial dipole layer is formed. In this work, we study methane on an Al(100) surface as a deliberately chosen model system for hybrid interfaces to evaluate the impact of this computational artifact. We show that changing the supercell size leads to artificial shifts in the calculated core-level energies that can be well beyond 1 eV for small cells. The same applies to atoms at comparably large distances from the substrate, encountered, for example, in extended, upright-standing adsorbate molecules. We also argue that the calculated work function change due to a core-level excitation can serve as an indication for the occurrence of such an artifact and discuss possible remedies for the problem.

The nuts and bolts of core-hole constrained ab initio simulation for K-shell x-ray photoemission and absorption spectra

“…Therefore, a uniform negative charge background is introduced into the calculation of the positively charged unit cell with the core-hole, which might lead to unpredictable changes in the calculated energies. This charging issue in periodic systems has previously been recognized as problematic [66,67]. The effect of the background charge can in principle be reduced by increasing the unit cell size.…”

“…The lack of charge neutrality, as is also the case for ΔSCF calculations of XPS binding energies, can provide a challenge for simulations of bulk and surface systems using PBC. While charge neutrality in the unit cell can be restored by the introduction of a homogeneous charge background [82], this charge background itself can lead to unphysical artefacts in the calculations [67]. Several solutions to this problem have been proposed [84,85], for example by placing the removed charge into the lowest unoccupied molecular orbital (LUMO) of the system as done in the XCH method which introduces a full core-hole [26,30]:…”

“…In the future we will provide a more systematic study of method differences on a larger set of benchmark systems, to test if the performance of the ΔIP-TP and ΔIP-XTP approaches is robust across a variety of cases including more challenging systems, e.g. containing coherent dipolar arrangements [67].…”

First-principles calculations of hybrid inorganic–organic interfaces: from state-of-the-art to best practice

“…For such systems the unit cells in periodic boundary conditions would have to be made enormously big to prevent finite unit cell size effects, which introduce spurious interactions (especially if the defects are charged). [18][19][20][21] In cluster calculations such interactions cannot exist by design.…”

“…4,8,10 As discussed above, cluster calculations without electrostatic embedding will fail to account for these effects for extended surfaces. Due to the periodic boundary conditions in repeated slab simulations, these effects 21 In such cases in which a periodicity of the interface is assumed that does not exist in the actual system, repeated slab calculations can produce even qualitatively wrong results.…”