Subject: conflict between energy and imaginary frequency

From: Karla Tersago <karla.tersago:at:ua.ac.be>
Date: Mon, 06 Jun 2005 11:34:44 +0200

Dear CCL'ers,

When I perform a geometry optimization, with frequency calculation of a 
certain molecule with Gaussian03, I have the following problem:
Cs symmetry: imaginary frequency of -20 cm-1, so this is transition 
state structure.
C1 symmetry: no imaginary frequencies, so it is a minimum on the 
potential energy surface. It's lowest frequency is 27 cm-1.
I checked the normal modes of both structures and the imaginary 
frequency of the Cs form and the lowest frequency of the C1 form 
correspond to the same normal mode.
The problem is that energy of the optimized structured shows that the Cs 
form has a lower energy (it is stabilized by 0.02 kcal/mol) than the C1 
form. 

How can it be that a transition state has a lower energy than a minimum? 

The calculations were done with B3LYP/6-311+G*
opt=verytight int=ultrafine freq 

As the data conflict, I thought it might be a problem with some 
thresholds, so I tried to set better thresholds with:
Iop(3/27=15) and Iop(3/29=15) 

I also tried iop(3/32=x) in combination with the above set options, but 
for x=3,4,5 the scf did not converged (even not if I put MaxCycle on 500). 

Does anybody have any suggestions on how I can solve this? 

Best regards,

Karla

-- 
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Karla Tersago
Structural Chemistry Group, Department of Chemistry
University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium
E-mail: karla.tersago:at:ua.ac.be /Phone:+32(03)8202366/Fax:+32(03)8202310 
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From: "Xavier ASSFELD" <Xavier.Assfeld -()- cbt.uhp-nancy.fr>
Date: Mon, 6 Jun 2005 17:04:05 +0200

Dear,

there are two possible explainations. The first one, as you mentioned is the
numerical noise due to the grid of integration. The cure is to use a
"better" grid! Is it possible?
The second possibility is intramolecular BSSE. Look at the work of Franck
Jensen Chem. Phys. Lett. 261 (1996) 633.
Yours.
                               ...Xav

Pr. Xavier Assfeld             Xavier.Assfeld -()- cbt.uhp-nancy.fr
Chimie et Biochimie theoriques T: (33) 3 83 68 43 82
Faculte des Sciences           F: (33) 3 83 68 43 71
54506 Vandoeuvre, France       http://www.lctn.uhp-nancy.fr

  -----Message d'origine-----
  De : Computational Chemistry List [mailto:chemistry-request -()- ccl.net]De la
part de Karla Tersago
  Envoye : lundi 6 juin 2005 11:35
  A : chemistry -()- ccl.net
  Objet : CCL:conflict between energy and imaginary frequency

  Dear CCL'ers,

  When I perform a geometry optimization, with frequency calculation of a
certain molecule with Gaussian03, I have the following problem:
  Cs symmetry: imaginary frequency of -20 cm-1, so this is transition state
structure.
  C1 symmetry: no imaginary frequencies, so it is a minimum on the potential
energy surface. It's lowest frequency is 27 cm-1.
  I checked the normal modes of both structures and the imaginary frequency
of the Cs form and the lowest frequency of the C1 form correspond to the
same normal mode.
  The problem is that energy of the optimized structured shows that the Cs
form has a lower energy (it is stabilized by 0.02 kcal/mol) than the C1
form.

  Best regards,

  Karla

--
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Karla Tersago
Structural Chemistry Group, Department of Chemistry
University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium
E-mail: karla.tersago -()- ua.ac.be /Phone:+32(03)8202366/Fax:+32(03)8202310
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