CCL:G: Problems finding TS

[Herbert Fruchtl <herbert.fruchtl%st-andrews.ac.uk>]

 Sent to CCL by: Herbert Fruchtl [herbert.fruchtl],[st-andrews.ac.uk]
 What I usually do in this case:
- Guess the length of the bond at the TS, or look for the maximum in a
 relaxed PES scan.
 - Optimize to a minimum with this bond frozen at this value.
- Start TS search from this point. If possible, calculate a Hessian
 (with a smaller basis and a pure DFT functional like HCTH or BLYP)
You have ModRedundant AND TS in your input. I don't know what you're
 freezing, but this won't give you a true TS. Also, if you are so far
 off
 that you need NoEigenTest, it's unlikely you find the correct TS.
   Herbert
 Debellis Anthony CE US anthony.debellis,,ciba.com wrote:
>  Sent to CCL by: Debellis Anthony CE US [anthony.debellisHello Oscar,
>  One possibility would be to locate the transition state with the methyl group
>  removed, then add back the methyl to the resulting TS structure followed by a
>  further TS optimization.
>  Anthony DeBellis
>  Ciba
>  -----Original Message-----
> > From: owner-chemistry^^ccl.net [mailto:owner-chemistry^^ccl.net]
> >  
>  Sent: Tuesday, December 18, 2007 12:05 PM
>  To: Debellis Anthony CE US
>  Subject: CCL:G: Problems finding TS
>  Sent to CCL by: Oscar Odio Chacón [odio---imre.oc.uh.cu]
>  Hello, CCLers:
> I´m trying to calculate the transition state of a radical
>  addition reaction to an olefin using Gaussian 98. The problem arises because the
>  output negative frequency does not correspond with vibrational motion of the
>  bond formed in the product, but corresponds to a motion associated with a methyl
>  group near the radical. So, what can I do?
>  Remove the methyl group?
>  In my opinion, this methyl group would have influence in the transition state
>  energy.
>  The overall system contains 47 atoms, 415 basis functions, 707 primitive
>  gaussians, 70 alpha electrons, 69 beta electrons. The command line was  #
>  b3lyp/6-31g(d,p) opt=(modredundant, TS, noeigentest) freq.
>  Thanks in advance.
>  Oscar Odio
>  --
>  Lic. Oscar Odio Chacón
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 Herbert Fruchtl
 EaStCHEM Fellow
 School of Chemistry
 University of St Andrews