Assessment of Functionals for TD-DFT Calculations of Singlet-Triplet Transitions.

The calculation of transition energies for electronically excited states remains a challenge in quantum chemistry, for which time-dependent density functional theory (TD-DFT) is often viewed as a balanced (computational effort/obtained accuracy) technique. In this study, we benchmark 34 DFT functionals in the specific framework of TD-DFT calculations for singlet-triplet transitions. The results are compared to accurate wave function data reported for the same set of 63 excited-states, and it turns out that, within the selected TD-DFT framework, BMK and M06-2X emerge as the most efficient hybrids. This investigation clearly illustrates that the conclusions drawn for singlet excited states do not necessarily hold for triplet states, even for similar molecular structures.