Conclusions
In this work, we have validated the applicability of density functional theory (DFT) methods to the prediction of the X-ray photoelectron spectra (XPS) of ionic liquids. On the example of data for forty ion pairs, we demonstrated how the core-level electron binding energy (BE) could be calculated and used to plot theoretical spectra at a low computational cost. The delta Kohn–Sham (ΔKS) method provides the most reliable results in comparisson to the experimental spectra. Negative 1s Kohn–Sham orbital energies show a strong correlation with the calculated ΔKS BE values. However, the sequence of orbital energies differs from the ΔKS BEs data. The correlation of the calculated atomic charges with the ΔKS BE values is notably weaker. Nonetheless, that correlation can be improved by accounting for the electrostatic potential due to the surrounding atoms in the ion pair. Thus, the XPS spectra can be predicted based on the DFT calculations using three different approaches. The choice between them implies a compromise between the computational cost and accuracy. Besides being more accurate, the ΔKS method is also more resource-demanding and less accessible than the other two methods. The use of 1s Kohn–Sham orbital energies and atomic charges are justified when qualitative aspects of X-ray photoelectron spectra are of higher interest than the absolute binding energy values or when the available computational power is limited.