Can TD-DFT calculations accurately describe the excited states Behavior of stacked nucleobases? The cytosine dimer as a test case

By using calculations rooted in the time dependent density functional theory (TD-DFT) we have investigated how the lowest energy excited states of a face-to-face pi-stacked cytosine dimer vary with the intermonomer distance (R). The perfomances of different density functionals have been compared, focussing mainly on the lowest energy single excited state of the dimer (S-1)(2). TD-PBE0, TD-LC-omega PBE, and TD-M05-2X provide a picture very similar to that obtained at the CASPT2 level by Merchan et al. (J Chem Phys 2006,125,231102), predicting that (S1)2 has a minimum for R similar to 3 angstrom, with a binding energy of similar to 0.5 eV, whereas TD-B3LYP, TD-CAM-B3LYP, and TD-PBE understimate the binding energy. However, independently of the functional employed, no low-energy spurious charge transfer transitions are predicted by TD-DFT calculations, also when a nonsymmetric dimer is investigated, providing encouraging indications for the use of TD-DFT for studying the excited state of pi-stacked nucleobases. (c) 2007 Wiley Periodicals, Inc.