Kasper Hald, Asger Halkier, Poul Jørgensen
Department of Chemistry, Århus University, DK-8000 Århus C, Denmark

Sonia Coriani
Dipartimento di Scienze Chimiche, Università degli Studi di Trieste via Licio Giorgieri 1, I-34127 Trieste, Italy

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

Trygve Helgaker
Department of Chemistry, University of Oslo, P. O. B. 1033, N-0315 Oslo, Norway

J. Chem. Phys. 118, 2985-2998 (2003).
(Received 5 August 2002; accepted 29 October 2003)

Using a Lagrangian formulation an integral-density direct implementation of the analytic CCSD(T) molecular gradient is presented, which circumvents the bottleneck of storing either O(N⁴) two-electron integrals or O(N⁴) density matrix elements on disk. Canonical orbitals are used to simplify the implementation of the frozen-core approximation and the CCSD(T) gradient is obtained as a special case. Also a new, simplified approach to (geometrical) derivative integrals is also presented. As a first application we report a full geometry optimization for the most stable isomer of SiC₃ using the cc-pV5Z basis set with 368 contracted basis functions and the frozen-core approximation.

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