CCL: Negative frequency problem

 Sent to CCL by: Marcel Swart [marcel.swart _ icrea.es]
On Jun 14, 2007, at 10:54 PM, Christopher Cramer cramer,,chem.umn.edu wrote:
There have been several posts recently with questions and proposed solutions to the problem of "too many negative frequencies" for transition-state (TS) structures.
So, what can one do? Some codes permit one to choose a finer quadrature grid, and this often does solve the problem. Of course, one can then worry about whether one should go back and recompute one's full set of stationary points with this finer grid (it's only computer time...) but at least one knows that the issue is not a chemical one. Another option is to compute the frequencies by finite difference of the first derivatives (probably even MORE expensive and not really an ideal option). Lastly, one can boldly rely on one's chemical intuition to know when one is being plagued by this problem and attempt to sleep well while blithely ignoring the issue (after all, the mode in question will have a "true" frequency that will contribute negligibly to zero-point vibrational energy, and negligibly to enthalpy, and will be so small that the correct value should not be used on the QMHO approximation for entropy in any case).
I would just like to add that ADF has the possibility to scan the normal mode, to verify the corresponding frequencies; with the 2006.01 version, this is being done by default for all imaginary frequencies that are found.
This option is related to the (so-called mode-tracking) frequency scanning by Reiher, Neugebauer and co-workers. 
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 dr. Marcel Swart
 ICREA researcher at
 Institut de Química Computacional
 Universitat de Girona
 Campus Montilivi
 17071 Girona
 Catalunya (Spain)
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 +34-972-418356
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 marcel.swart_-_icrea.es
 marcel.swart_-_udg.edu
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 http://iqc.udg.edu/~marcel
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