TD-DFT calculations are afflicted by severe underestimation of excitation energies for states having chargeseparated character, as has been widely discussed. In a sufficiently large system, this can manifest as "spurious" CT states, 1,24-28 whose energies are much lower than what one would estimate using Mulliken's formula, ω CT (R) ≈ IE + EA + 1 4πǫ 0 R . (6.1) Equation (6.1) expresses the excitation energy ω CT between well-separated donor and acceptor moieties in terms of the ionization energy (IE) of the donor and the electron affinity (EA) of the acceptor, along with a Coulomb penalty of 1/(4πǫ 0 R) for creating an ion pair. In conjugated polymers, problems with underestimated CT energies in TD-DFT lead to over-delocalization of the exciton wave functions. The phenomenology of this problem is discussed next, in Section 6.1. We have already seen (in Section 5.3) that one can design metrics to measure the degree of CT in a given excited state. These and related metrics are discussed in the context of TD-DFT's CT problem in Section 6.2.