Subject: Re: CCL:M:Calculation of entropy

From: [email-masked] (Frederic BOUYER)
Date: Wed, 20 Sep 1995 09:21:13 +0100

Hi Victor,

I would say that in general I had the same problems with negative
frequencies to calculate vibrational contributions to thermochemical
functions. I used DMol and DGauss, and I tried always to get all
frequencies, even those of small energy. Why?

First, did you try to change (or pertubate) your geometry in order to fully
re-optimize your structure?
Did you optimize with strong Keywords (PRECISE, small gradients ...)
Generally speaking, I would say you got the wrong geometry and you had a
local maximum (a so called transition state with one negative frequency or
a local maximum of the order n if you have n negative frequencies).

Second, I would say that frequencies of small energy are **very** important
in vibrational contributions because of the analytical expression ! Small
frequencies of the order of 50 cm-1 (with my clusters) completely change
thermochemical functions (S, H, and G(T), and DeltaG(T) of reaction ) if
you either not take them into account or replace them by 35 cm-1 or 65 cm-1
!
But for frequencies that are normally in the order of 200 cm-1, they not
strongly influence thermochemical functions.
See how frequencies change thermodynamical quantities with your molecules.

If you are interested in more comments, send me an email.
You would probably be interested by (and references therein) :

F. Bouyer, G. Picard and J. J. Legendre, "Computational chemistry : a way
to reach spectroscopic and thermodynamic data for exotic compounds", to be
published in Journal of Chemical Information and Computer Science
[special issue of the ICCCRE XI (XIth International Conference on Computers
in Chemical Research and Education, Paris, France, July 17-21), 1995.]

Hope this helps.

Frederic Bouyer

____________________________________________________________________
Frederic BOUYER
Laboratoire d'Electrochimie et de Chimie Analytique
Equipe "Reactivite en Milieux Ioniques Liquides" (G. PICARD)
ENSCP
11, rue Pierre et Marie Curie
75231 PARIS Cedex 05
FRANCE
Tel : (33)-1-43-54-53-84 ou (33)-1-44-27-67-51 ou (33)-1-44-27-66-94
Fax : (33)-1-44-27-67-50
E-mail : [email-masked]
http://alcyone.enscp.jussieu.fr/  created, in progress ...
http://alcyone.enscp.jussieu.fr/Pages/LECA/GP/FB/