  First principles simulation of x-ray photoemission spectroscopy (XPS) is an
important tool in the challenging interpretation and assignment of XPS data of
metal-organic interfaces. We investigate the origin of the disagreement between
XPS simulation and experiment for the azulene molecule adsorbed on Ag(111). We
systematically eliminate possible causes for this discrepancy, including errors
in the structural model and finite size effects in periodic boundary
conditions. By analysis of the electronic structure in the ground-state and the
core-hole excited-state, we are able to trace the error back to artificial
charge transfer between adsorbed molecule and metal surface. This is caused by
the self-interaction error of common exchange-correlation functionals. This
error is not remedied by standard hybrid or range-separated hybrid functionals.
We employ an ad hoc self-interaction error correction based on molecular
orbital projection, that exposes this issue and is able to recover the correct
experimental behaviour. The charge transfer artefact also negatively affects
the prediction of X-ray absorption spectra for this system. Both the simulated
photoemisson and x-ray absorption spectra show a better agreement with
experimental data, once the ad hoc correction is employed. Similar
core-hole-induced charge artefacts may affect core-level simulations at
metal-organic interfaces more generally.
