Analytic gradients for excited states in the coupled-cluster model CC2 employing the resolution-of-the-identity approximation

Andreas Köhna and Christof Hättigb

a Universität Karlsruhe (TH), Institut für Physikalische Chemie, Lehrstuhl für Theoretische Chemie, Kaiserstra&sslig;e 12, D-76128 Karlsruhe, Germany
b Forschungszentrum Karlsruhe, Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany

J. Chem. Phys. 119, 5021-5036 (2003).
(Received 13 May 2003; accepted 11 June 2003)

The derivation and implementation of excited state gradients is reported for the approximate coupled-cluster singles and doubles model (CC2) employing the resolution-of-the-identity approximation for two-electron repulsion integrals. The new code is profiled for a set of examples with up to 1348 basis functions and exhibits no I/O-bottlenecks. A test set of sample molecules is used to assess the performance of the CC2 model for adiabatic excitation energies, excited state structure constants and vibrational frequencies. We find very promising results, especially for adiabatic excitation energies, though the need of a single-reference ground state and a single-replacement dominated excited state poses some limits on the applicability of the method. Its reliability, however, can always be tested on grounds of some diagnostic measures. As an example application, we present calculations on the π* ← π excited state of trans-azobenzene.

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