The spatial overlap between particle and hole densities defines a region of space corresponding to a localized excitation. In contrast, charge separation suggests that the product ∆ρ elec (r) ∆ρ hole (r) ≈ 0, i.e., there is little or no spatial overlap between the excited electron and the hole. This is depicted schematically in Fig. using electron and hole densities that are drawn to resemble the Gaussian approximations in Fig. . Based on this idea, Etienne et al. suggest a charge-separation metric based on this product: φ = ∆ρ elec (r) ∆ρ hole (r) .24), using cartoon sketches of particle and hole densities made to resemble those in Fig. . Roughly speaking, the integrand in Eq. (5.24) is non-zero in the blue region of overlap between ∆ρ elec (r) and ∆ρ hole (r). (b) Examples of a localized excitation (on the left) and a CT excitation (on the right) in the push-pull system (H2C)2N-(C6H4)5-NO2. In its planar geometry (on the left), there is significant overlap between ∆ρ elec (r) and ∆ρ hole (r), corresponding to a large value of φ, but when the molecule is twisted (on the right), these densities localize on opposite ends of the molecule and φ is small. Calculations in (b) were performed at the TD-PBE0/6-311++G(2d,p) level and the plots are adapted from Ref. 13.