In the canonical MO basis, the same transition is completely scrambled and essentially uninterpretable. Two of the relevant canonical virtual orbitals are shown in Figs. 5b and 5c but there are 17 distinct amplitudes with |x ia | ≥ 0.1, the largest of which contributes only 19% of the norm of the transition eigenvector. and all of which collectively contribute only 85%. Note that Fig. indicates the fraction of |ψ(r)| 2 that is encapsulated within each isosurface, which is necessary in order to make meaningful side-by-side comparisons of orbital localization or size. Isoprobability surfaces are be readily computed, given volumetric data on a grid (e.g., in the format of a "cube" file), and this functionality is available in some visualization programs. It is thus conceivable that x is characterized by just one singular value with λ 2 1 ≈ 1. In such cases, the principle NTO pair can be used to distill the picture into one that involves just one occupied and one virtual orbital, without loss of information. Such is also the case in the S 0 → S 2 transition of the fluorononeterminated polyfluorene that is shown in Fig. , for which λ 2 1 = 0.96. In contrast to the simple picture in the NTO basis, in the canonical MO basis four different amplitudes x ia contribute significantly to the same transition. The latter are highly delocalized in the occupied space and form a localized hole only upon coherent superposition, whereas the localization is immediately evident in the NTO basis. Higher-lying states of polyfluorenes do involve a larger number of significant NTO pairs, which is not unusual. Since NTOs are the optimal particle/hole basis, the presence of more than one significant singular value λ i is a signature of unresolvable multideterminant character in the excited-state wave function, which cannot simply be rotated away by unitary change of basis. The next section considers this in more detail.