Note how off-diagonal elements of Ω (or ∆P) characterize coherences between atoms or fragments in the electronic excitation. A value L coh = 1 indicates no off-diagonal contributions to Ω, which implies either that the excitation is localized on a single site or else that it is a superposition of localized excitations, i.e., a FE state. (Consult the schematic Ω matrices in Table .) The length scale over which the FE state is delocalized is measured in the extracule coordinate and a sensible definition of a PR for this is 186