TDDFT Assessment of Functionals for Optical 0–0 Transitions in Small Radicals

Using LR-TDDFT, we calculated the 0–0 energies of 15 small radicals for which the experimental values in gas phase are available. We used 17 functionals. It turned out that B3LYP, M06-2X, ωB97X-D, CAM-B3LYP, and HSE06 functionals are the most effective functionals in terms of root-mean-square and average unsigned deviation. Using the standard value (0.47 a0–1) of the attenuation parameter ω, the long-range-corrected LC-GGA functionals give poor results. However, the LC-PBE with ω = 0.25 a0–1 give a performance similar to that of B3LYP. Taking into account zero-point correction improves the results, but the contribution of adiabatic correction is more important than that due to the vibration. The vertical approximation is certainly not recommended. An adiabatic calculation seems to give a good compromise between computing time (and resources) and reliability of the results for most of molecules investigated in this work.