Heike Fliegl^a, Christof Hättig^b and Wim Klopper<sup>a

a</sup> Universität Karlsruhe (TH), Institut für Pysikalische Chemie, Lehrstuhl für Theoretische Chemie, Kaiserstrasse 12, D-76128 Karlsruhe, Germany
^b Forschungszentrum Karlsruhe, Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany

J. Chem. Phys. 122, 084107 (2005).
(Received 10 November 2005; accepted 29 November 2004)

A simplified singles-and-doubles linear-r₁₂ corrected coupled-cluster model, denoted CCSD(R12), is proposed and compared with the complete singles-and-doubles linear-r₁₂ coupled-cluster method CCSD-R12. An orthonormal auxiliary basis set is used for the resolution-of-the-identity approximation to calculate three-electron integrals needed in the linear-r₁₂ Ansatz. Basis-set convergence is investigated for a selected set of atoms and small molecules. In a large basis, the CCSD(R12) model provides an excellent approximation to the full linear-r₁₂ energy contribution, whereas the magnitude of this contribution is significantly overestimated at the level of second-order perturbation theory.

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