Subject: RE: Transition state-imaginary frequency

From: Stephen Bowlus <[email-masked]>
Date: Tue, 8 Oct 2002 15:04:56 -0700

There are several old threads on this subject from which you can extract
details ...

Having a single imaginary frequency ensures that you are at a saddle point
in the energy surface, which corresponds to a transition state.

The significance of more than one imaginary frequency has been debated on
this page.  There seem to be two schools of thought:  1) if there is one
large frequency (say, greater than 100 cm-1) and all others are small (say,
less than 10 cm-1), this is good enough, and don't worry about it; 2) the
additional imaginary frequencies may have some physical significance, so
they should always be eliminated, regardless of magnitude.

How to do this may depend on the software that you use.  The manual for
MOPAC6 has a writeup on the problem (see the section on "escaping from a
hilltop").  I have found a couple of shortcuts which usually resolve the
issue in MOPAC: 1) change the optimization method (e.g. from eigenvector
following to Bartel's method) or 2) re-refine the structure, starting from
an output file (not a restart file).  The perturbation induced by the
rounding/truncation in writing out the output is sometimes enough to permit
further refinement.

Cheers,
Steve

> -----Original Message-----
> From: Duc Nguyen [mailto:[email-masked]]
> Sent: Tuesday, October 08, 2002 2:14 PM
> To: [email-masked]
> Subject: CCL:Transition state-imaginary frequency
> 
> 
> Dear CCLers,
> 
> I accidentally ran by your comments in CCL.net about the transition
> state should have one and only one imaginary frequency. Please suggest
> me some references to read more about the matter as why it is true and
> how to eliminate the other imaginary frequencies. I would greatly
> appreciate your help.
> 
> Regards,
> Duc
> 
> 
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