Subject: transition state

From: "tianxiao young" <[email-masked]>
Date: Wed, 08 Aug 2001 03:32:19 +0000

Dear CCLers,

Do you know the best way to calculate the optimization transtion state using 
Gassian98? Your kind help is most appreciated.

txyoung

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From: elewars <[email-masked]>
Date: Wed, 08 Aug 2001 17:21:09 -0400

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

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tianxiao young wrote:

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