is the RMS exciton size. The metrics d CD1 , d CD2 and d CD3 are explored for the first time in the present work, and d CD1 proves to be particularly useful as an invariant replacement for Γ. Note that d CD1 and other expectation values introduced in this section are no more expensive to compute than ∆r or ∆σ, as they require the same TD-DFT amplitudes and one-electron integrals. They are even less expensive to calculate than Λ, which requires a quadratic number of numerical quadrature steps in order to evaluate the integrals O ia .