Subject: Transition State

From: Phil Hultin <[email-masked]>
Date: Thu, 9 Aug 2001 08:47:08 -0500 (CDT)

We prefer to do TS optimizations in G98 using either the QST2 or QST3
approaches, rather than the "classical" OPT=TS approach suggested by
another CCL'er.

You can read about the procedure for submitting these jobs in the Gaussian
manual, and there is discussion in the "blue book" by Foresman and Frisch
that comes with Gaussian.

The problem with OPT=TS is that your guessed structure has to be VERY good
or you will fall off of the saddle point.  With very simple structures it
isn't hard to set up a reasonable guess, but if you are probing more
complex systems it can be a problem.  The QST2 approach asks for optimized
geometries for the starting and ending structures, and then it
interpolates between them to obtain a guess.  The QST3 approach asks for
both starting and ending geometries plus a guess at the TS geometry.  Both
of these are much more robust than OPT=TS.

Do not forget to run a frequency calculation on the geometry that results
> from any TS search to check that it displays one and only one imaginary
frequency.  Also, you must check that the atomic displacments
corresponding to this frequency actually do lie along the reaction
coordinate you are interested in.  You can do this by inspection of the
frequency output, but I find it easier to use a program such as MOLEKEL
that can animate frequencies.

Dr. Philip Hultin
Associate Professor
Dept. of Chemistry
University of Manitoba
Winnipeg, MB Canada
[email-masked]

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From: elewars <[email-masked]>
Date: Thu, 09 Aug 2001 16:47:19 -0400

Hello,

I usually submit a guess to Spartan and request an AM1 TS opt; this very rarely
fails: Spartan's TS algorithm does not seem to need a *very* good guess. I then
submit the AM1 TS to a HF/3-21G TS opt., usually with Spartan The resulting
structure almost always works for a higher-level TS opt with G98 (I suspect the
AM1 structure would be about as good for a starting geometry). Of course there
may be families of TS's that do not lend themselves to initial guessing and
require the automated procedure.

EL
====

Phil Hultin wrote:

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From: Christoph van Wüllen <[email-masked]>
Date: Thu, 9 Aug 2001 18:25:47 +0200

>interpolates between them to obtain a guess.  The QST3 approach asks for

If you form a bond, the "optimized geometry" of the educt may be very far
away. In this case a constrained optimization with the "bond" distance fixed
to value of, say, 3 Angstrom for a C-C-bond to be formed, should be done.
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