Can TD-DFT calculations accurately describe the excited states 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 {\bf (S$_1$)$_2$}. TD-PBE0, TD-LC-PBE0 and TD-M05-2X provide a picture very similar to that obtained at the CASPT2 level by Merchan et coll. (J. Chem. Phys 2006, 125, 231102), predicting that {\bf (S$_1$)$_2$} has a minimum for R$\sim$3$\AA$, with a binding energy of $\sim$0.5 eV, whereas TD-B3LYP, TD-CAM-B3LYP and TD-PBE understimate the binding energy. However, independently of the functional emploied, no low-energy spurious Charge Transfer transitions are predicted by TD-DFT calculations, also when a non-symmetrical dimer is investigated, providing encouraging indications for the use of TD-DFT for studying the excited state of $\pi$-stacked nucleobases.