Subject: Coupled Cluster calculations (CCD,CCSD) in G94

From: Pablo Vitoria Garcia <[email-masked]>
Date: Tue, 20 May 1997 15:13:43 +0100 (WET DST)

Hi,

In a recent paper (J. Chem. Phys. 1997, 106(15), 6430) CCSD(T) is found 
to be a very reliable model to calculate accurate molecular structures, 
even better than MP4 or CISD, for molecules containing first-row atoms.

When I tried to optimize a molecule at the CCSD(T) level, I found out 
that G94 cannot do that, since there are no analytical gradients 
available for that method. Instead I tried CCD, although I guess it's not 
so accurate as CCSD. But the wavefunction didn't converge after 50 
iterations. So there go my questions:

1. Is there any way (perhaps a non-standard route) to perform an 
optimization at the CCSD level with the available numerical gradient?

2. The convergence criterion at the beginning of the CCD calculation:

   Iterations=  50 Convergence= 0.100D-07

   what does it refere to? Can I lower it without affecting too much the 
reliability of the calculations?

Thanks a lot for your help

Pablo

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Pablo Vitoria Garcia 
Departamento de Quimica Inorganica, Facultad de Ciencias
Universidad del Pais Vasco (UPV/EHU)
Apartado 644, E-48080 Bilbao
SPAIN
e-mail: [email-masked]
Phone: +34 4 4647700 Ext. 2450
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