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.