Results are plotted along the bottom of Fig. for the acenes, (PPV) n , and O 2 N(Ph) n NH 2 systems. For the acenes, exciton delocalization (as measured by d rms ) continues to increase through the largest size examined, with no indication of of a plateau (Figs. 3i). This indicates complete delocalization and we believe this behavior is consistent with ω GDD values that continue to decrease at least through n = 20 (see Fig. ), despite the aforementioned ground-state instability. The behavior of d rms is quite different for the (PPV) n systems, as shown in Fig. . Regardless of which tuning procedure is used, the exciton size reaches an asymptotic value by n = 9 or 10, although that value differs by about 1.1 Å depending on whether ω IE or ω GDD is used. This is consistent with as asymptotic excitation energy that is reached for the same oligomer size (Fig. ), and indicates a maximum value for the effective conjugation length. Notably, GGA functionals afford complete exciton delocalization for (PPV) n , rather than a finite exciton size. This is inconsistent with correlated wave function calculations, whereas LRC functionals afford the correct result. To this, we can add that the larger exciton size that is obtained using IE tuning, meaning that d rms (ω IE ) > d rms (ω GDD ) for a given oligomer size, is consistent with the fact that ω GDD > ω IE (Fig. ). The latter fact implies that the onset of exact exchange occurs on a shorter length scale for GDD tuning as compared to IE tuning, restriction the delocalization of the former relative to the latter. However, this distinction comes with only a modest difference in the S 1 excitation energies.