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Mail:
Dept. of Chemistry
Ohio State University
100 W. 18th Ave.
Columbus, OH 43210

Office:
412 CBEC

Email:
herbert@
chemistry.ohio-state.edu

The electronic spectrum of alternant polycyclic aromatic hydrocarbons (PAHs) includes two singlet excited states that are often denoted ¹L_a and ¹L_b. Time-dependent density functional theory (TD-DFT) affords reasonable excitation energies for the ¹L_b state in such molecules, but often severely underestimates ¹L_a excitation energies and fails to reproduce observed trends in the ¹L_a excitation energy as a function of molecular size. Here, we examine the performance of long-range-corrected (LRC) density functionals for the ¹L_a and ¹L_b states of various PAHs. With an appropriate choice for the Coulomb attenuation parameter, we find that LRC functionals avoid the severe underestimation of the ¹L_a excitation energies that afflicts other TD-DFT approaches, while errors in the ¹L_b excitation energies are less sensitive to this parameter. This suggests that the ¹L_a states of certain PAHs exhibit some sort of charge-separated character, consistent with the description of this state within valence-bond theory, but such character proves difficult to identify a priori. We conclude that TD-DFT calculations in medium-size, conjugated organic molecules may involve significant but hard-to-detect errors. Comparison of LRC and non-LRC results is recommended as a qualitative diagnostic.