CCL:conflict between energy and imaginary frequency

 Dear Karla and Xav,
If the energies that Karla is talking about include zero point energy, then this is a common issue. I have seen the potential energy + zpe for a transition structure exceed that of a close-by minimum at least a couple of dozen times (out of several thousand calculations). The issue is that the zpe for the minimum has one extra contributing normal mode compared to the transition structure, and this adds to its zpe. In reality, the zpe is being wrongly calculated by the harmonic approximation in these cases. However, under any circumstances, one should not take too seriously a minimum that is in such an exceedingly shallow well. I don't see how BSSE can cause what was observed. I suppose an integration grid issue might, in principle, though I've never ran into that with an ultrafine grid. Finally, make sure all of the cartesian forces on the 'optimzed' structures are small - in my experience for simple structures, about 1% of optimizations in reduntant internal coordinates lead to structures that are not really fully optimized, even with opt=verytight. 
 Dan Singleton
  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[at]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[at]ccl.net]De la
  part de Karla Tersago
   Envoye : lundi 6 juin 2005 11:35
   A : chemistry[at]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.
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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