Can TD-DFT predict excited states in endoperoxides?

The performance of Time-Dependent Density Functional Theory (TD-DFT) in the prediction of the low-energy region of the electronic absorption spectrum of a model aromatic endoperoxide (anthracene-9,10-endoperoxide, APO) has been assessed against accurate MS-CASPT2//CASSCF transition energies and oscillator strengths for comparison. This survey includes functionals belonging to one of the four main categories: GGA, meta-GGA, Global Hybrids and Range-Separated Hybrids. Our results show that the absorption spectrum of APO is strongly functional-dependent. In general, GGA and meta-GGA functionals poorly describe charge transfer (CT) excitations but reasonably reproduce local π*σ* and ππ* transitions. The incorporation of a fixed or increasing fraction of exact exchange along interelectronic distances improves the description of CT states. A functional, which combines a 32% of exact exchange, 72% of non-local B88 exchange and LYP correlation is found to be the best choice, with an average error of 0.75 eV. Despite this relatively large error, this functional is able to reproduce correctly the order of the lowest π*σ* and ππ* excited states of the UV spectrum of APO.