Quantifying electron delocalization is a long-standing problem, and many indices have been developed. Among them, the one most extensively used is the delocalization index (DI). Despite its accuracy for the prediction of aromaticity, its physical legitimacy has been debated. On the other hand, a recently derived electron delocalization range (EDR) 151 function appears not to have such limitation and may be used to connect the FO theory with delocalization in chemical systems. For details on DI and EDR and discussing the latter's advantages, please refer to the Appendix A.2.