Phys. Chem. Chem. Phy., 10, 4119-4127 (2008).
Received 4th March 2008, Accepted 15th April 2008
First published as an Advance Artical on the web 5th June 2008
A generalization of the spin-component scaling and scaled opposite-spin modi. cations of secondorder Moller-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 Moller-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.†Present addres: COSMOlogic GmbH & Co. KG, Burscheider Strasse 512, D-51381 Leverkusen, Germany.
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