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

From: Jan Hrusak <[email-masked]>
Date: Thu, 22 May 1997 11:48:46 +-200

Dear Pablo, 

You may have also a look to our recent paper on the comparisons of all the 
CC based methods (CCD, CCSD, CCSD(T), BD, BD(T), QCISD, QCISD(T))
in J. Chem. Phys. 1997, 106, 7185. 
In general the Gaussian allows you to do numerical gradient calculations 
just using the opt command. The number of iterations can be increased 
by typing the IOP(9/6=xxxx) option. 

Jan Hrusak

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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