Subject: RE: transition state

From: "Shobe, Dave" <[email-masked]>
Date: Thu, 09 Aug 2001 09:08:24 -0400

This is a good response.  A few things I would add (E. Lewars may or may not
agree):

1. I would recommend calcHFFC even for HF level calculations (calcFC is
equivalent in this case).  Alternatively, if you preoptimized at a lower
level like HF/3-21G, you can do a freq job at that level and then use readFC
instead.  Finding a TS is so much easier with an accurate Hessian (and even
then it can be difficult!)  

2. When a starting geometry for a TS is unavailable, the algorithms
mentioned are requested using Opt=QST2 and opt=LST.  You can also do a
"relaxed scan" of a reaction coordinate, provided you can approximate it
with a single geometric parameter, using opt=modredundant.  I will give
further instructions if needed--the instructions in the manual are kind of
overwhelming since there are so many variations in the use of modredundant.

--David Shobe

-----Original Message-----
From: elewars [mailto:[email-masked]]
Sent: Wednesday, August 08, 2001 5:21 PM
To: [email-masked]
Subject: CCL:transition state

re txyoung, TS calcs:

First try to get a reasonable input geometry. I usually guess it (I find
this
almost always works), build it with an interacive molecule builder like
GaussView or Spartan's builder, then do an AM1 or HF/3-21G TS optimization,
then
submit the AM1 or 3-21G TS structure to your ab initio or DFT calc.
With Gaussian I usually use Opt(TS, CalcHFFC, NoEigenTest, MaxCycle=99)

The CalcHHFC (calc Hartree-Fock Force constants) is to get a good Hessian if
you
are doing a correlated-level calc (e.g. MP2); for a HF-level calc omit it.
NoEigenTest tells Gaussian not to check the eigenvalues of the Hessian as it
goes along; Gaussian TS calcs always seem to fail if you omit NoEigenTest
(at
least G94 did). MaxCycle=99 is so a difficult job won't fail just because it
ran
out of optimization cycles.

It may be helpful to get an AM1 or HF/3-21G TS using some other program,
then
feed the Cartesians into Gaussian.

If you have problems guesssing a starting geom, Gaussian has algorithms for
getting one from the reactant and product structures--look in help under
optimization options.

E. Lewars

=================
tianxiao young wrote:

> Dear CCLers,
>
> Do you know the best way to calculate the optimization transtion state
using
> Gassian98? Your kind help is most appreciated.
>
> txyoung
>
> _________________________________________________________________
> Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp
>

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From: elewars <[email-masked]>
Date: Thu, 09 Aug 2001 16:37:20 -0400

Yes, CalcFC may well help for a TS opt (but probably not for a PES-minimum opt),
as it is likely to give a better Hessian than the default molecular
mechanics-type Hessian.

A PES scan could be helpful in difficult cases

EL
======