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J Chem Theory Comput. 2014 Mar 11;10(3):1035-47. doi: 10.1021/ct4009975.

Electronic Energy Gaps for π-Conjugated Oligomers and Polymers Calculated with Density Functional Theory.

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Department of Chemistry, University at Buffalo, State University of New York , Buffalo, New York 14260-3000, United States.
Shanghai Key Laboratory of Advanced Polymeric Materials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology , Shanghai 200237, P. R. China.


In varying contexts, the terms "energy gap" (energy difference) or "band gap" may refer to different experimentally observable quantities or to calculated values that may or may not represent observable quantities. This work discusses various issues related to calculations of electronic energy gaps for organic π-conjugated oligomers and linear polymers by density functional theory (DFT). Numerical examples are provided, juxtaposing systematic versus fortuitous agreement of orbital energy gaps with observable fundamental (ionization vs electron attachment) or optical (electronic excitation) energy gaps. Successful applications of DFT using nonempirically tuned hybrid density functionals with range-separated exchange (RSE) for calculations of optical gaps, fundamental gaps, and electron attachment/detachment energies are demonstrated. The extent of "charge-transfer like" character in the longest-wavelength singlet electronic excitations is investigated.


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