Subject: Re: CCL:Converge problem in the optimization on G94

From: <[email-masked]>
Date: Tue, 25 Jul 1995 09:35:42 -0400 (EDT)

> I use the gaussian 92 and g94 programs. I have a problem about the geometry
> optimization
> and frequency analysis.
> After the geometry optimization completed ,I
>performed the frequency calculation analysis. However, from the output of frequency
>analysis ,
> sometimes Optimization is not completed because values about displacement
> exceed the thresholds. How should I resolved it and wolud you tell me your
>thought about this problem ?

I believe the standard solution to this problem is to use "opt=CalcAll"
instead of "freq" to calculate the frequencies.  This way, the geometry
will be refined if the forces are above threshold, and you will also
get the frequency results.

Good luck!

Karl I.

----------------------------------------------
Dr. Karl Irikura
Chemical Kinetics and Thermodynamics Division
National Institute of Standards and Technology
Gaithersburg, MD  20899
----------------------------------------------

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From: Anthony P Scott <[email-masked]>
Date: Wed, 26 Jul 1995 09:36:48 +1000

A little time ago a person asked why his optimization ran fine but when a
subsequent frequency on the optimized stucture was ran the final geometry check
revealed that some of the convergence criteria failed.

There is possible two reasons (maybe more) for this.

1. the optimized structure was arrived at by an APPROXIMATE updated hessian. This
means that when the frequency is ran, which producesd the accurate hessian,

gaussian finds that some of the criteria are not met and you end up with some
NO's.  The solution here is to either run the optimization again with
opt=tight (as one person has already suggested) or to look at the trivial
frequencies in the frequency log file and determine if thay are too large
for you liking. If they are small (maybe < 20cm-1, they should however by
zero) then it is likely that the frequencies are not in error by any 
significant degree (in my experience).

2. The second reason for the convergence criteria not to be met in the
frequency log file is if the optimization was run with some constraints. The
frequency is performed in the full 3n-6 space, regardless of anf constraints
that you may have imposed in the optimization.  The solution here is of
course to remove the geometry constraints.

I hope this is of some help to all.

Kind Regards,

Anthony P. Scott
Research Officer
Computational Chemistry Group
Research School of Chemistry
Australian National University
Canberra, ACT, Australia