Binding energy correlation with orbital energy and atomic charges
Recently it was proposed that the BEs obtained from fitting of the experimental XPS spectra for ILs correlate with the calculated atomic charges.14,15 Figure 6 demonstrates a modest correlation between the BE approximated by the first and third terms of Eq. 3 (BE = V (qi )) and the calculated relative ΔKS BE values. The relation improves significantly by considering the effect of the electrostatic potential of neighbouring atoms given by the second term in Eq. 3 (BE ≈V (qi ,qj )). Consideration of the neighbour atoms increases the coefficient of determination (R 2) for individual ion pairs (from 0.04–0.65 to 0.35–0.97) as well as for the whole set of forty ion pairs (from 0.28 to 0.83). Nevertheless, the deviations allow the difference in the order of BEs calculated via ΔKS and V (q ) methods.
It should be noted that the V (q ) method has an excessive cost in comparison to the ε(1s) method – it calls for more resources to perform the charge analysis. For example, for the studied ion pairs, the DDEC6 analysis is almost as demanding as a single point DFT calculation. On the bright side, charge analysis does not require all-electron basis sets, allowing the application of pseudopotentials and effective core potentials to recoup the cost of any post-analysis.