To quantify the extent of electron delocalization and its effect on reactivity, we employed both an orbital-based and a density-based approach. Firstly, we calculated the occupation number (Nocc) of the natural bond orbital (NBO) corresponding to the breaking bond, where deviation from a full occupation of 2 (denoted 2-Nocc) describes the degree of bond delocalization (i.e., 𝜒 = 2-Nocc). For example, electron donation from a (breaking) C-C s bond into a geminal s* orbital in cyclopropane increases the value of 𝜒, capturing the hyperconjugation (delocalization) effect proposed by Weinhold and Landis (Figure ). We also computed the 𝜒 = Ds/Ds 0 ratio used to calculate the electron localization function (ELF = (1 + 𝜒 2 ) -1 ), which measures the excess kinetic energy density due to Pauli repulsions (Ds) relative to the uniform electron gas, Ds 0 . ELF values close to 1 correspond to highly localized electrons, while values closer to 0 indicate complete delocalization. The close agreement between these parameters suggests that the effect of delocalization on reactivity, described by the term 𝜒, is correctly captured. In summary, we therefore expect a decrease in ∆E ‡ either through an increase in driving force (𝛼 > 0, as predicted by the BEP principle), and/or an increase in bond delocalization (β < 0).