is the x component of r elec , for example. The quantity d - e-h is is the distance between the centroids of ∆ρ elec (r) and ∆ρ hole (r), or in other words, the norm of the expectation value of the intracule coordinate r elec -r hole . It is the same quantity introduced in eq. 1, and we henceforth omit the superscript and define