Benchmarking the performance of spin-component scaled CC2 in ground and electronically excited states
A generalization of the spin-component scaling and scaled opposite-spin modifications of second-order Møller–Plesset perturbation theory to the approximate coupled-cluster singles-and-doubles model CC2 (termed SCS-CC2 and SOS-CC2) is discussed and a preliminary implementation of ground and excited state energies and analytic gradients is reported. The computational results for bond distances, harmonic frequencies, adiabatic and 0–0 excitation energies are compared with experimental results to benchmark their performance. It is found that both variants of the spin-scaling increase the robustness of CC2 against strong correlation effects and lead for this method even to somewhat larger improvements than those observed for second-order Møller–Plesset perturbation theory. The spin-component scaling also enhances systematically the accuracy of CC2 for 0–0 excitation energies for π → π* and n → π* transitions, if geometries are determined at the same level.