Roughly speaking, this integrates over the blue region in Fig. , corresponding to overlap of ∆ρ elec (r) and ∆ρ hole (r). In Eq. (5.24), we omit the normalizing denominator that is included in the definition of Ref. 12, as it equals unity for TD-DFT calculations where both ∆ρ elec (r) and |∆ρ hole (r)| integrate to exactly one electron.