Transition moments and excited state first-order properties in the second-order coupled cluster model CC2 using the resolution of the identity approximation

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

Andreas Köhn
Institut für Physikalische Chemie, Universität Karlsruhe, D-76128 Karlsruhe, Germany

J. Chem. Phys. 117, 6939-6951 (2002).
(Received 19 June 2002; accepted 23 July 2002)

An implementation of transition moments and excited state first-order properties is reported for the approximate coupled cluster singles and doubles model (CC2) using the resolution of the identity (RI) approximation. In parallel to the previously reported code for the ground and excited state amplitude equations, we utilize a partitioned form of the CC2 equations and thus eliminate the need to store any N4-scaling intermediates. The accuracy of the RI approximation is tested for a set of 29 molecules for the aug-cc-pVXZ (X = D,T,Q) basis sets in connection with the recently optimized auxiliary basis sets. These auxiliary basis sets are found to be sufficient even for the description of diffuse states. The RI error is compared to the usual basis set error and is demonstrated to be insignificant.

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