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A Lagrangian, integral-density direct formulation of the analytic CCSD and CCSD(T) gradients

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(*N*^{4})
two-electron integrals or O(*N*^{4}) 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 SiC_{3} using the cc-pV5Z basis set
with 368 contracted basis functions and the frozen-core approximation.

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