Time-Dependent Density-Functional Description of the 1La State in Polycyclic Aromatic Hydrocarbons: Charge-Transfer Character in Disguise?

详细信息    查看全文

• 作者：Ryan M. Richard ; John M. Herbert
• 刊名：Journal of Chemical Theory and Computation
• 出版年：2011
• 出版时间：May 10, 2011
• 年：2011
• 卷：7
• 期：5
• 页码：1296-1306
• 全文大小：1207K
• 年卷期：v.7,no.5(May 10, 2011)
• ISSN：1549-9626

文摘

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.