Time-dependent density functional theory for radicals: An improved description of excited states with substantial double excitation character

Spin-unrestricted time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies for the first several excited states of four diatomic radicals and methyl, nitromethyl, benzyl, anilino, and phenoxyl radicals. While the excitation energies of Rydberg states computed by TDDFT are significantly inferior to the results of Hartree–Fock-based single excitation theories, TDDFT performs slightly better than the HF-based theories for valence states with dominant single excitation character. For valence states with substantial double excitation character, TDDFT is a striking improvement over the HF-based theories, the latter being qualitatively incorrect descriptions for these states.