Excitation Energies of Stacked DNA Base Pair

The excitation energies of uracil dimers have been calculated with the configuration interaction singles (CIS) method, the configuration interaction with single excitations with perturbed doubles (CIS(D)) method, the equation-of-motion coupled-cluster method with singles and doubles excitations (EOM-CCSD), and the time-dependent density-functional theory (TDDFT) with pure and hybrid functionals. It is shown that the charge transfer excitations behave as 1/R at large molecular distance limit and the local excited states are split by the so-called electronic couplings. The calculated excited energies by the ab initio methods (CIS, CIS(D), EOM-CCSD) well reproduce those features. But the results of TDDFT cannot show those features because of the short-sightness of exchange correlation functionals of DFT. The inclusion of the electron correlation is crucial to the excitation energies, especially to the charge transfer excitation, of which the excitation energies are decreased by more than 2 eV.