Finally, it is worth noting that (∆r) NTO ≈ d e-h (see Table ). For a transition that is dominated by a single principal NTO pair, it follows that the quantity (∆r) NTO does measure electron-hole separation. The same cannot be said for (∆r) CMO or (∆r) Boys , even if λ 2 1 ≈ 1. Furthermore, we find that (∆σ) NTO is a good approximation to |σ elec -σ hole |, which makes sense in terms of the definition of ∆σ (eq. 11) and implies that Γ NTO ≈ d CD1 when λ 2 1 ≈ 1. This is clear in the data provided in Table , where Γ NTO = 2.69 Å as compared to d CD1 = 2.72 Å. This provides a rationale for the stability of Γ NTO that was observed but not explained in Ref. 11.