High-level theoretical benchmark investigations of the UV-vis absorption spectra of paradigmatic polycyclic aromatic hydrocarbons as models for graphene quantum dots.

Five paradigmatic polycyclic aromatic hydrocarbons (PAHs) (pyrene, circum-1-pyrene, coronene, circum-1-coronene, and circum-2-coronene) are used for studying the performance of three single-reference methods {scaled opposite-spin-algebraic diagrammatic construction to second-order [SOS-ADC(2)], time-dependent (TD)-B3LYP, and TD-Coulomb-attenuating method (CAM)-B3LYP} and three multireference (MR) methods [density functional theory/multireference configuration interaction (DFT/MRCI), strongly contracted-n-electron valence state perturbation theory to second order (NEVPT2), and spectroscopy oriented configuration interaction (SORCI)]. The performance of these methods was evaluated by comparison of the calculated vertical excitation energies with experiments, where available. DFT/MRCI performs best and thus was used as a benchmark for other approaches where experimental values were not available. Both TD-B3LYP and NEVPT2 agree well with the benchmark data. SORCI performs better for coronene than for pyrene. SOS-ADC(2) does reasonably well in terms of excitation energies for smaller systems, but the error increases somewhat as the size of the system gets bigger. The natural transition orbital analysis for SOS-ADC(2) results indicated that at least two configurations were essential to characterize most of the lower-case electronic states. TD-CAM-B3LYP gives the largest errors for excitation energies and also gives an incorrect order of the lowest two states in circum-1-pyrene. A strong density increase of dark states was observed in the UV spectra with increasing size except for the lowest few states which remained well separated. An extrapolation of the UV spectra to infinite PAH size for S1, S2, and the first bright state based on the coronene series was made. The extrapolated excitation energies closest to experimental measurements on graphene quantum dots were obtained by TD-CAM-B3LYP.