(4.26) where the λ i are the singular values associated with each NTO pair. The quantity PR NTO is a participation ratio in the NTO basis; cf. Eq. (4.18). For the six-unit polymer described in Table , the quantity PR NTO starts at a value of 1.6 for the S 1 state and increases monotonically as one moves up the excitation manifold, with PR NTO ≥ 4 for states S 5 and S 6 . This means that S 5 and S 6 are each characterized by an average of four significant particle/hole pairs, indicating significant static correlation. With this in mind, it is interesting to revisit the NTOs in Fig. , which correspond to an excited state of a different polymer and where three pairs of NTOs are needed to recover 81% of the transition density. Interestingly, these NTOs demonstrate that the excitation delocalizes around the bent portion of the molecule, suggesting that a purely geometric definition of broken conjugation is insufficient to understand exciton localization in these molecules (and similarly inadequate to define the effective size of the chromophore), because other mechanisms such as dipole-dipole coupling and superexchange can drive delocalization even when geometric distortion leads to loss of conjugation. With an excited-state wave function in hand, however, an effective chromophore size can be inferred by measuring the particle-hole separation for the exciton. This is the anti-diagonal coordinate in the Ω plots of Table , for a different PPV system. This analysis technique and other statistical measures of electron-hole correlation are described in the next section.