Subject: RE:Summary of gibbs free energy

From: "GAO Yi" <[email-masked]>
Date: Sat, 15 Mar 2003 08:09:57 +0800 (HKT)

Dear Everyone,

Thank you for your help.Below are the summary of the response for my
questions.

Gao Yi

Dear Gao,
I have not much experience in the field of computational chemistry, but
maybe I can be of help. The FREQ keyword in G98 yields gibbs corrections,
that are only accurate as long as:
-the normal modes you are considering are quite harmonic (otherwise,
the partition function for this mode is not going to be very good)
-the normal modes have a frequency high enough, that doesn't require a
hindered rotor treatment.
(I'm suposing here that the "non-local minimum" you are speaking of is the
variational transition state theory minimum, sorry if I am mistaken and
this answer is out of the point).
Anyway, there is on the net (I don't remember where I printed this
from) a paper called  "Thermochemistry in Gaussian", that might  be
interesting.
Regards,

Olalla Nieto Faza
Dpto. Qu¨ªmica Org¨¢nica
Universidade de Vigo
Lagoas-Marcosende 36200 Vigo
[email-masked]

On Tue, 11 Mar 2003, GAO Yi wrote:

> Dear CClers,
>
> I'm now calculating the gibbs energy via Gaussian98. I wonder whether I
> can use the FREQ keyword to get the gibbs corrections for the non-local
> minimum and whether these results are correct to represent the points of
> the Gibbs free energy surface.
>
> Gao Yi
>
>
>
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Dear Gao,
The calculation of frequencies in no-minimun-energy points is used, for
example, when following the Minimum energy path (in order to sample the
gibbs energy surface). The problem (of course) is that the first
derivatives are not zero, so you have to project the frequencies that are
not the frequency you are following (=the one that corresponds to the
reaction coordinate) so that they are ortogonal to the gradient vector.
This can be done with the GAMESS package (though I haven't used it), I
don't know if G98 has this option implemented.
(I think Truhlar has written about this kind of things if you need further
reference; there are some programmes of his group Polyrate and Gaussrate
that do this kind of variational studies, but here ends all my expertise
in the subject).
Regards

Olalla Nieto Faza
Dpto. Quimica Organica
Universidade de Vigo
Lagoas-Marcosende 36200 Vigo (Spain)
e-mail: [email-masked]

No, but take a look at freq=projected for special cases,
e.g. points on a carefully optimised reaction path,
inflection points on surface...

Hope this helps,

J.
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  Short answer is no.  The thermodynamic corrections involve zero-point
energies computed from the vibrational frequencies and temperature
corrections from the partition functions.  Both of these presume that
the gradient is zero in the analysis which results in frequencies and at
arbitrary points on the surface this is not true.

  There are special cases such as along a reaction path, where the forces
can be projected out and the structure minimized wrt all other coordinates
and the FREQ=Projected option can be used.  But for arbitrary points, while
the data is printed it is not rigorously correct.

>
> Dear CClers,
>
> I'm now calculating the gibbs energy via Gaussian98. I wonder whether I
> can use the FREQ keyword to get the gibbs corrections for the non-local
> minimum and whether these results are correct to represent the points of
> the Gibbs free energy surface.
>
> Gao Yi
>
>
>
> -= This is automatically added to each message by mailing script =-
> [email-masked] -- To Everybody  | [email-masked] -- To Admins
> Ftp: ftp.ccl.net  |  WWW: http://www.ccl.net/chemistry/   | Jan:
[email-masked]

-- 

  Douglas J. Fox
  Technical Support
  Gaussian, Inc.
  [email-masked]