Subject: Summary: Rotation Warning message G98

From: "john thaler" <jdthlr/at/hotmail.com>
Date: Tue, 20 May 2003 14:23:21 -0300

Thank you to all who replied to this.  Here are the two (2)responses I 
received:

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these 15 degrees of freedom certainly correspond to pseudo-rotation of OH
group (since you are studying a sugar molecule...) The potential energy 
profil
of this rotation looks like a "sinusoode" (sine function). In the harmonic
approximation
the shape of the potential energy surface is supposed to be a parabola. 
Hence
it is difficult to think that a parabola can simulate a sine function...
You can circumvent this problem with the Hindered Rotation keywords in G98.
Hope this helps.

--

                                      ...Xav

Pr. Xavier Assfeld                          Xavier.Assfeld/at/lctn.uhp-nancy.fr
Laboratoire de Chimie thiorique     (T) 33 3 83 68 43 74
Universiti Henri Poincari                (F) 33 3 83 68 43 71
F-54506 Nancy B.P. 239              http://www.lctn.uhp-nancy.fr

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I'm not sure about the first message but the second indicates the molecule
has 15 vibrational normal modes with small frequencies.  These modes
(probably torsions) really shouldn't be treated as harmonic oscillators in
the thermodynamic analysis that follows the Zero point calculation --
rather they are hindered rotors which have a different and substantially
more complicated statistical mechanical contribution to the entropy and
specific heat.

Should you ignore them?  Depends what you are using the calculation for.
If all you want is the structure you can completely ignore them.  If you
want a low to medium quality estimate of the thermodynamics of the molecule
or of a reaction involving the molecule I would still ignore them.  If you
want a very high quality estimate of the thermodynamics you should consider
doing something more sophisticated -- but I warn you it is a lot of tedious
work to do this "properly".  Gaussian does include a hindered rotor
facility (FREQ=HINDROT) which I have not used.  Check the manual.

Rob Waterland
DuPont
Robert.L.Waterland/at/usa.dupont.com

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