Although NTOs provide the most compact basis in which to visualize an excitation, it cannot be assumed that an arbitrary transition is dominated by a single pair of NTOs. From Tozer's data set that was used in Section 4 .2, the 1 B 3u state of the linear acenes is a good example where there are two significant eigenvalues of ∆P elec with comparable magnitudes. Since the eigenvalues λ 2 i are related directly to natural occupation numbers, 2,99 this indicates unresolvable mul- ticonfigurational character in the excited state, 2 which can occur even when the ground state is comfortably single-reference. This section will explore several such examples, including the linear acenes (Section 4 .3 .1) but also examples involving conjugated polymers (Section 4 .3 .2) and an example with multiple electronicallycoupled chromophore units (Section 4 .3 .3).