Benchmarking coupled cluster methods on singlet excited states of nucleobases.

In this paper, coupled cluster methods CC2, CCSD, CCSDR(3) and EOM-CCSD(T) have been benchmarked against CC3 for the transition energies of nucleobases. Beside presenting vertical excitation energies for about 30 singlet transitions of four molecules, the results are analyzed statistically and problematic cases have been discussed in detail. It is concluded that the mean deviation of the CC2 results is smaller than that of the CCSD. However, the latter seems to be more systematic, i.e. it usually overestimates excitation energies by about 0.2 eV but with somewhat smaller standard deviation. Unfortunately, with decreasing single excitation contribution in the wave function CCSD gives large error, which can not be corrected by the non-iterative triples methods CCSDR(3) and EOM-CCSD(T).