Adélia J. A. Aquino^1,2,*, Dana Nachtigallova³, Pavel Hobza³, Donald G. Truhlar⁴, Christof Hättig⁵, Hans Lischka1,3,*
¹Institute for Theoretical Chemistry, Währingerstrasse 17, University of Vienna, A-1090 Vienna, Austria

²Institute of Soil Research, University of Natural Resources and Life Sciences Vienna, Peter-Jordan-Stra&sslig;e 82, A-1190 Vienna, Austria

³Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic and Center for Biomolecules and Complex Molecular Systems, Flemingovo nam. 2, 166 10 Prague 6, Czech Republic

⁴Department of Chemistry and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455-0431

⁵Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44801 Bochum, Germany

J. Comput. Chem. 32, 1217-1227 (2011).
Received 9 August 2010; Revised 27 September 2010; Accepted 1 October 2010; Published online: 29 November 2010

Keywords: charge transfer; excited states; coupled cluster; DFT; stacked nucleobases

Electronic singlet excitations of stacked adenine-thymine (AT) and guanine-cytosine (GC) complexes have been investigated with respect to local excitation and charge-transfer (CT) characters. Potential energy curves for rigid displacement of the nucleobases have been computed to establish the distance dependence of the CT states. The second-order algebraic diagrammatic construction [ADC(2)] method served as reference approach for comparison to a selected set of density functionals used within the time-dependent density functional theory (TD-DFT). Particular attention was dedicated to the performance of the recently developed family of M06 functionals. The calculations for the stacked complexes show that at the ADC(2) level, the lowest CT state is S₆ for the AT and as S₄ for the GC pair. At the reference geometry, the actual charge transferred is found to be 0.73 e for AT. In case of GC, this amount is much smaller (0.17 e). With increasing separation of the two nucleobases, the CT state is strongly destabilized. The M06-2X version provides a relatively good reproduction of the ADC(2) results. It avoids the serious overstabilization and overcrowding of the spectrum found with the B3LYP functional. On the other hand, M06-HF destabilizes the CT state too strongly. TD-DFT/M06-2X calculations in solution (heptane, isoquinoline, and water) using the polarizable continuum model show a stabilization of the CT state and an increase in CT character with increasing polarity of the solvent.

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