CC3 triplet excitation energies using an explicit spin coupled excitation space

Kasper Hald
Department of Chemistry, Århus University, DK-8000 Århus C, Denmark

Christof Hättig
Forschungszentrum Karlsruhe, Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany

Jeppe Olsen and Poul Jørgensen
Department of Chemistry, Århus University, DK-8000 Århus C, Denmark

J. Chem. Phys. 115, 3545-3552 (2001).
(Received 6 December 2000; accepted 5 June 2001)

Triplet excitation energies are derived in the approximate triples model CC3 using an explicit spin coupled triplet excitation space. The explicit spin coupled excitation space gives considerable computational savings compared to the spin-orbital approach. Sample calculations are performed on the Be₂ and CH₂ systems and the performance of the CC3 results are evaluated from comparison with full configuration interaction (FCI) results.

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