The metric φ is defined such that 0 ≤ φ ≤ 1. If φ = 0 then there is no overlap between electron and hole, meaning that the excitation in question is entirely CT-like. An actual example that lies close to that limit is the end-to-end donor-acceptor electron-transfer excitation of the twisted push-pull chromophore (H 3 C) 2 N-(C 6 H 4 ) 5 -NO 2 , whose particle and hole densities are plotted on the right of Fig. . A twist in the geometry severs the conjugation of the π system, resulting in particle and hole densities that localize on opposite ends of the molecule and a much smaller value of φ as compared to when the molecule is planar. When planarity is restored (on the left in Fig. ), both ∆ρ elec (r) and ∆ρ hole (r) delocalize across the entire π system, leading to a value of φ that is closer to its upper limit.