Linear response CC2 triplet excitation energies

We report an atomic integral direct implementation of the approximate coupled-cluster singles and doubles method CC2 for the triplet excitation energies of closed shell molecules. Employing an explicitly spin-coupled triplet excitation space we obtain a compact and efficient formulation which needs approximately the same operation count as required for singlet excitation energies. Compared to a spin orbital based approach this gives a reduction by about a factor of 3. A basis set study employing basis sets with up to 432 functions is presented for the lowest triplet excitations in benzene and the results are compared with other theoretical and experimental results.