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

From: Pablo Vitoria Garcia <[email-masked]>
Date: Tue, 27 May 1997 15:30:33 +0100 (WET DST)

Hi,

Thanks to everyone who helped with my question about CC optimisations in 
G94. All the answers are summarized below, following my message. In a few 
words:

  - CCSD optmizations can be done numerically in G94.
  - Although the convergence criterion can be changed, it seems 
to a better choice to increment the number of allowed cycles.
  - CCSD analytical optimizations can be done with ACESS II, which is 
higly recomended over G94 for CC claculations in general.

My question was:

> 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?

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From: "Michael Nolan (WannaBe Quantum Chemist)" <[email-masked]>

You could simply try and increse the number of iterations from 50
the convergence criteria refer simply to the maximum number of
iterations(50) and what the difference between two successive iterations
(0.100D-07) must be.
I don't know too much about G94, our own copy of it still isn't working
(even Gaussian Inc. couldn't get it to work!), so I cannot be too
specific.

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From: Qiang Cui <[email-masked]>

If possible, do NOT use Gaussian for CCSD etc. calculations. It's very
slow compared to Molpro or ACES. Both of them can handle CCSD(T) very
efficiently. However, as far as I know, ACES is the only one that can
actually handle CCSD(T) analytical gradient. They have implemented
analytical hessian as well for CCSD(T). (and all other approximations as
well as excited state extensions.)

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From: "Eric V. Patterson" <[email-masked]>

G94 can indeed optimize molecules at the CCSD(T) level, just not
analytically.  This makes the optimization very slow, or course.
Frequency calculations become even more painful.  

You may set the convergence threshold by specifying CCSD(T,conver=N),
where N is the desired threshold, 10^-N.  I would not be inclined to
reduce this figure too far below the default of 8, maybe no lower than 6.

You don't mention what starting geometry you are using.  It would be
advisable to start with a good MP2 of DFT structure.  This may help with
some of your convergence difficulties.

Finally, a personal aside.  I use CCSD(T) frequently.  It is a very
reliable, high-level method.  However, I don't use Gaussian for it.  The
ACES II package from the Quantum Chemistry Program at the University of
Florida (http://qtp.ufl.edu/Aces2) is a robust program designed
specifically for coupled-cluster applications.  Optimizations are done
analytically.  It is no more difficult to use than Gaussian, and is
superior for CC calculations, in my opinion.

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From: Doug Fox <[email-masked]>

   Gaussian 94 can optimize structures with CCSD and CCSD(T) but since
only energy calculations are available it will use numerical gradients
instead.  For CCD there are analytic gradients so the normal optimizer
will be used.

   For numerical gradients the gradient is only as accurate as the energy
which is computed.  So while you can cut down the convergence with
the Conver=N option the possible accuracy of your structure is directly
affected.  If the convergence is close with 50 cycles you can use the
MaxCyc=M option to increase the number of iterations in the CC.  For
CCD I would highly recommend against reducing these because the optimization
assumes that it will get at least the default accuracy in the gradients.

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From: Hui-Hsu Tsai <[email-masked]>

   I think the program "PSI" can do CCSD(T) geometry optimization.

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From: Jan Hrusak <[email-masked]>

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. 
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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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