Accurate excitation energies from time-dependent density functional theory: assessing the PBE0 model for organic free radicals

A spin-unrestricted approach to time-dependent density functional theory is applied to calculate vertical excitation energies of some small and medium-size organic radicals, including nitromethyl, benzyl, anilino and phenoxyl. The computational model is obtained combining the Perdew–Burke–Erzenrhof (PBE) generalized gradient functional with a predetermined amount of exact exchange. Our results show that this method (PBE0) provides accurate excitations both to valence and low-lying Rydberg states, even in the presence of substantial contributions from double excitations.