As with any empirical model, there is an inherent limitation to the accuracy achievable with the 'strain/delocalization' components from which it derives, since other factors, such as dipole effects and non-covalent interactions present at the TS but not in the reactant or product, and explicit variation in bond force constants, are neglected. Such additional factors could be incorporated into a linear free energy relationship through the inclusion of further descriptors. However, we feel that the overall improvement in barrier height prediction (R 2 = 0.85, RMSE = 1.5 kcal mol -1 , Figure ) compared with the BEP model (R 2 = 0.67, RMSE = 2.3 kcal mol -1 , Figure ) illustrates the generality and importance of delocalization on reactivity across a range of organic reactions without recourse to a deeper analysis of multiple factors. It is also instructive to compare the results of conventional DI-AS analysis to the complementary strain/delocalization model introduced here: the more delocalized a breaking bond is, the less such a bond is required to distort to adopt the TS geometry, leading to an earlier TS. Likewise, greater delocalization could facilitate stronger electronic interaction between reactants due to enhanced orbital overlap earlier along the reaction coordinate. One drawback of the DI-AS approach is the necessity for explicit knowledge of the TS geometry and energy; in contrast, the strain/delocalization model introduced in this work enables a quick and quantitative estimation of reactivity using solely ground state properties. This feature is anticipated to be valuable when designing new 'strain-release' driven reactions.