Subject: irc summary

From: "Szieberth Denes" <[email-masked]>
Date: Tue, 9 Dec 1997 13:30:08 +0000

Here' s a summary of the replies I've got to my question concerning
Gaussian IRC calculations. Thanks for all who have replied.

 My original question was:

> Several of my internal reaction coordinate -following optimum searches
> run into the same problem: from a TS which has a negative frequency
> apparently corresponding to a dissociation, the resulting minima in both
> direction are the same. Has anyone experienced similar problems? Could it
> be a problem of the IRC method implemented in gaussian? I'd be grateful for
> any kind of help.

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did you use something like opt=(tight,ts)? if you don't use tight
then g94 sometimes think that forward and reverse are the
same direction. actually what happens is that your ts search
doesn't end up at the ts itself but only near the ts. if
the distance of your apparent ts structure to the actual ts
is smaller than the stepsize of the irc than the irc will not
cross the barrier and 'roll back'. in this case this is neither
a bug in g94 nor a bug in your input but just an insufficient
optimisation.
hope this helps

Ernst-Udo Wallenborn
Phys. Chem. Lab

---------------------------------------------------------

Hello!

I am not sure this is the case with your calculations, but the
symptoms are the same; sorry if this is wrong. If you have a
molecule with some symmetry (for example, Cs), and the vibrational
vector corresponding to the imaginary frequency is not of full-symmetric
representation (A" for Cs), then your structures should be the same.
Could it be your problem?

I saw you saying "apparently correcponding to a dissociation". I assume
you looked up your vector; please let me know when you find out what
was the problem -- I am extensively using IRC, so I want to be aware
of any possible bug.

Thanks.
Dmitry Khoroshun
[email-masked]

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1)  I've found the IRC routines in G94 to work properly only from version
D1 onward (see CCL archives for more information on this issue).  For
earlier versions the answer might be:  upgrade.
2)  To help the IRC it sometimes helped me to take big stepsizes in the
beginning.  Otherwise the IRC 'got stuck' around the TS.  I have not
encounterd your problem yet, but might it be related to the flatness of the
PES around the TS?

I look forward to a summary of the answers (hoping that some real IRC-gurus
will also respond)

Best regards,

Han

______________________________________________________________________
Han Zuilhof                         |  E-mail:  [email-masked]
Laboratory of Organic Chemistry     |  FAX   :  31-317-484914
Department of Biomolecular Sciences |  phone :  31-317-482367
Wageningen Agricultural University  |
Dreijenplein 8                      |
6703 HB  Wageningen                 |    "Excite a photochemist!"
The Netherlands                     |

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I had a similar problem last year, which was never completely resolved as I
moved before completing the project.  You may be able to resolve this by
optimizing with "verytight" convergence criteria, and using a small step
size for the IRC search.  You could also try contacting the developer of
the IRC method, Prof. Schlegel, at [email-masked]

Jaime Martell, Ph.D.                    P.O. Box 2317
Camille and Henry Dreyfus Fellow        Batesville, AR 72503-2317
Biology and Chemistry Division  Phone:  870-698-4688
Lyon College                            Fax:  870-698-4622

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	Eventually the usage of bigger stepsize, and in some cases simply the
application of cartesian coordinates instead of the default mass-weighted
coordinates have solved my problem.

        Denes Szieberth
        Dep. of Inorg. Chem.
        Technical University of Budapest
        Hungary
        [email-masked]