title: Benchmarking the performance of spin-component scaled CC2 in ground and electronically excited states.
abstract: 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 pi --> pi* and n --> pi* transitions, if geometries are determined at the same level.
authors.affiliation: Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, D-44780, Bochum, Germany.
authors.forename: Arnim
authors.initials: A
authors.lastname: Hellweg
authors.forename: Sarah A
authors.initials: SA
authors.lastname: Grün
authors.forename: Christof
authors.initials: C
authors.lastname: Hättig