The resolution to the DMABN paradox described above was made possible through the use of the first CT metric to be introduced for CT calculations, by Tozer and co-workers. Their proposed metric, denoted by Λ, is defined as Note the absolute value signs in the integrand of Eq. (6.4), which are necessary because occupied and virtual MOs are orthogonal. For this reason, we resist using the term "overlap" when it comes to MOs, in favor of "spatial proximity" versus "spatial separation", although we might call O ia the "spatial overlap". This and similar metrics are sometimes used to quantify the spatial proximity of HOMO and LUMO in donor-acceptor materials. In view of the normalization condition for x and y [Eq. (2.4)], it is unclear why the definition of Λ in Eq. (6.2) does not involve both x + y and xy. Perhaps it is in loose analogy to the expressions for the particle and hole density matrices [Eq. (2.11)], which contain terms like (x + y) † (x + y) and (x + y)(x + y) † , although these expressions also contain (xy) † (xy) and (xy)(xy) † . (The latter have sometimes been erroneously omitted in our own work as well. ) Whatever the reason, the definition of Λ in Eq. (6.2) is the one that has been used in practice, yet the decision to abdicate the proper normalization seems questionable, and has implications for other CT metrics that are discussed below. Within the TDA (where y = 0) there is no issue, as the denominator in Eq. ( .2) has a well-defined normalization, namely, ia x 2 ia = 1. The implies that 0 ≤ Λ ≤ 1 within the TDA, however this need not be the case in full TD-DFT. In practice, Tozer et al. find that 0.45 ≤ Λ ≤ 0.89 for localized valence excitations whereas Rydberg excitations lie in the range 0.08 ≤ Λ ≤ 0.27. Examining excitation energy errors as a function of Λ, it becomes clear that there are approximate (albeit functional-dependent) thresholds below which TD-DFT results should not be trusted. For example, when Λ < 0.4 (for B3LYP) or Λ < 0.3 (for PBE), the excitation energies are "likely to be in very significant error". Values of Λ are also found to correlate well with excitation energy errors along the torsional coordinate of a hydroxynaphthalene bichromophore that has the potential for intramolecular CT. The Λ metric has been successfully used to explain other trends in TD-DFT errors, although other metrics can be used in that capacity as well. Resolution of the DMABN paradox comes in noting that its intramolecular CT excitation at the PBE level exhibits Λ = 0.72 in the planar geometry, which is not very CT-like in real space. For the LRC-ωPBE/6-31G* calculations in Fig. , the corresponding values are Λ(S 1 ) = 0.53 and Λ(S 2 ) = 0.67 in the planar geometry, indicating that the nominal CT state actually has somewhat larger spatial proximity between particle and hole. This is actually true in the twisted geometry as well, although the values are much smaller and lie in the "danger zone": Λ(S 1 ) = 0.20 and Λ(S 2 ) = 0.22. Results at the TD-B3LYP/6-31G* level are similar.