For S 0 → S 3 , even the principle NTOs are delocalized over the length of the molecule (Fig. ), meaning that this is genuine delocalization and is not an artifact of the representation. The principle NTO pair accounts for only 67% of the transition density, with a second NTO pair contributing another 20%. Irreducible mixing of more than one NTO pair is a signature of static correlation in the excited state. (Note that there is no contradiction with the use of a single-determinant formalism for the ground state, because the CIS wave function ansatz is multideterminantal.) From another point of view, the presence of more than one significant singular value in the TDM indicates that the natural orbitals of the ground state differ significantly from those in the excited state. A close examination of the NTOs in Fig. reveals that ψ hole 1 (r) and ψ hole 2 (r) are out of phase with one another, on the left end of the molecule, but evolve across its length such that they are in phase on the right end of the molecule. The same is true of ψ elec 1 (r) and ψ elec 2 (r), which suggests that the excited state in question can only properly be described using a minimum of two determinants. This would not be obvious from attachment/detachment density analysis.