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

Abstract

X-ray photoemission (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy play an important role in investigating the structure and electronic structure of materials and surfaces. Ab initio simulations provide crucial support for the interpretation of complex spectra containing overlapping signatures. Approximate core-hole simulation methods based on density functional theory (DFT) such as the delta-self-consistent-field (ΔSCF) method or the transition potential (TP) method are widely used to predict K-shell XPS and NEXAFS signatures of organic molecules, inorganic materials and metal-organic interfaces at reliable accuracy and affordable computational cost. We present the numerical and technical details of our variants of the ΔSCF and TP method (coined ΔIP-TP) to simulate XPS and NEXAFS transitions. Using exemplary molecules in gas-phase, in bulk crystals, and at metal-organic interfaces, we systematically assess how practical simulation choices affect the stability and accuracy of simulations. These include the choice of exchange-correlation functional, basis set, the method of core-hole localization, and the use of periodic boundary conditions (PBC). We particularly focus on the choice of aperiodic or periodic description of systems and how spurious charge effects in periodic calculations affect the simulation outcomes. For the benefit of practitioners in the field, we discuss sensible default choices, limitations of the methods, and future prospects.

Topology Effects in Molecular Organic Electronic Materials: Pyrene and Azupyrene**

“…The calculated data for the UPS and NEXAFS spectra was rigidly shifted to match the experimental energy scale. The calculated NEXAFS transitions have already been published in context of method development [19] …”

“…The intensities for each transition are calculated as the magnitude of the transition dipole matrix elements. The overall spectrum is obtained by shifting the atomic NEXAFS contributions according to their ΔSCF binding energies following the so‐called ΔIP‐TP method [19,40a] …”

Physically inspired deep learning of molecular excitations and photoemission spectra

“…While we have only shown the application of this model for frontier orbital and quasiparticle energies, we are condent that it will be similarly applicable to the prediction of core-levels and X-ray photoemission signatures. 6,41 The ability to efficiently predict molecular resonances at high accuracy is key to enable large-scale computational screening of novel acceptor and donor molecules to be used in organic electronics and thin lm device applications. 7,81,84 We expect that the presented method will be very useful in this context.…”

“…While we have only shown the application of this model for frontier orbital and quasiparticle energies, we are confident that it will be similarly applicable to the prediction of core-levels and X-ray photoemission signatures. 6,41 …”

“…These fundamental molecular resonances associated with electron addition and removal in matter can be studied with photoemission and inverse photoemission spectroscopy. 6,7 However, the search for optimal materials combinations is limited by the speed at which organic materials combinations can be spectroscopically characterized. This is exacerbated by the challenge of interpreting macroscopically averaged photoemission data for complex molecules.…”