Both ∆r and d e-h are intended to measure electronhole separation but only the latter is invariant to unitary transformations of the MOs. Like ∆r, however, d e-h vanishes in any centrosymmetric system and this may cause interpretative problems, e.g., for symmetric or nearsymmetric arrangements of molecular chromophores. (The change in dipole moment is proportional to d e-h and also vanishes for centrosymmetric charge-resonance states, even when the forward and reverse CT contributions are individually large.) To circumvent this limitation, the quantity ∆σ was added to ∆r (eq. 10) to indicate size disparity between the donor and acceptor orbitals. Invariant metrics along the same lines are