Integral direct calculation of CC2 excitation energies: Singlet excited states of benzene

The calculation of excitation energies has been implemented in the CC2 model using an atomic integral direct algorithm where integrals are generated in distributions with one atomic index fixed and three indices free. The calculations are dominated by the recalculation of atomic integrals. Singlet excitation energies have been calculated for benzene and a basis set analysis has been carried out with basis sets containing up to 432 basis functions. The CC2 excitation energies have been compared with other theoretical and experimental results. Basis set effects of the order of 0.2 eV are found in benzene going from augmented double-zeta basis sets to augmented triple-zeta quality. The differences between the CC2 results and results obtained by the CASPT2 method are of the same order of magnitude as the basis set errors except for the 2 (1)E(2g) state. For this state, large discrepancies are found between the results obtained by coupled cluster methods and CASPT2.