The quantity d + e-h in Eq. (5.13) is the average of the extracule coordinate r elec + r hole for the electron/hole pair. This value depends on the choice of laboratory-fixed coordinate frame but can be used to assess how the exciton migrates upon change in molecular geometry. The average inter-particle (intracule) distance is d - e-h (Fig. ), however this vanishes for any centrosymmetric system, 18 meaning that it cannot detect charge separation in any system with inversion symmetry. This can have important implications in solidstate systems, where d - e-h (and correspondingly, the dipole moment change upon excitation) is zero or small, yet there may still be significant charge separation. In view of this, the quantity d exc in Eq. (5.12) is a more robust measure of electron-hole separation. The latter satisfies the bounds