The influence of correlation on (de)localization indices from a valence bond perspective

When going beyond the Hartree-Fock level to correlated methods, one observes a significant reduction in the delocalization index. This is commonly interpreted as a weakening of electron sharing due to electron correlation, although this is rather counter-intuitive to the concomitant energy lowering. In this study, we use an analytical valence bond model and full CI calculations to show that this reduction in the delocalization index actually goes hand in hand with increased covalent contributions at the expense of ionic contributions. This suggests that we should be careful in formulating interpretations of these results in (de)localization indices. Graphical Abstract Variation of the localization Δ(Ω_A, Ω_A) and delocalization index Δ(Ω_A, Ω_B) as a function of the parameter ω. By adjusting this parameter ω from [Formula: see text] to 0, we can gradually change the underlying wave function from a Hartree-Fock to a Heitler-London description.