# CCL: Negative frequency problem

*From*: Marcel Swart <marcel.swart-$-icrea.es>
*Subject*: CCL: Negative frequency problem
*Date*: Fri, 15 Jun 2007 13:50:40 +0200

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.
` ===================================
dr. Marcel Swart
ICREA researcher at
Institut de Química Computacional
Universitat de Girona
Campus Montilivi
17071 Girona
Catalunya (Spain)
fax
+34-972-418356
e-mail
marcel.swart_-_icrea.es
marcel.swart_-_udg.edu
web
http://www.icrea.es/pag.asp?id=Marcel.Swart
http://iqc.udg.edu/~marcel
===================================