* herbert@chemistry.ohio-state.edu relates the highest occupied molecular orbital (HOMO) energy level to the IE. By tuning ω in order to satisfy eq. 3, accurate CT excitation energies can be obtained. This paradigm has been so successful that some have described the description of CT states in TD-DFT as a solved problem. The IE-tuning (or "optimal" tuning 20 ) procedure described above is appealing because it is motivated by an exact condition. It starts from a model with correct asymptotic shape, since υ xc (r) ∼ -1/r for any LRC functional with 100% long-range Hartree-Fock exchange, as in eq. 1. Then, the correct asymptotic value υ xc (∞) is enforced via eq. 3. We rewrite the latter condition as