Subject: Summary2 : Frequencies calculated

From: "Emadeddin Tajkhorshid" <[email-masked]>
Date: Mon, 8 Jun 1998 14:12:16 -0700

Hi again

I have got two more respnses to my question which I will enclose here. Thanx
for your consideration. BTW, any further coments are still welcome:-)
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Regarding your query about geom vs freq accuracy. Since the calculation of the
freq. depend on the second derivative of the energy and how the basis set
describes the anharmonicity of a bond stretch ( or bend ), there is not so
simple as to compare with the exp. spectra and then say that the geom. is
accurately described. More often there are problem with getting some calc.
freqs to correlate well with spectra and others worse, which ones can change
with the basis set and theory used, which again leaves you with the problem of
which level of calc. to choose for the "correct" geometry...

But, and I guess that this is more important, I believe that in _most_ cases (
I have no refs or reasons ) the geom. is OK, when the calc. freqs. are...if you
get other replies saying differently I'm willing to reconsider

best wishes

Patrik

--
Dr Patrik Johansson
Inorg. Chem. Angstromlaboratory
Box 538 SE-751 21 Uppsala
Uppsala University SWEDEN
[email-masked]

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If you have used harmonic approximation to calculate frequensies from
force fiels and space structure in this case
in first level of approach the bond deformation (bending) are not
depended from the interatomic distances. This fact is due the kinematic
coefficient for this type of vibration [k=(1/m1+1/m2), where m1 and m2
are atomic masses] is not depended from the interatomic distances. For
other type of vibration the kinematic coefficients are depended from the
interatomic distances but these dependences are so complicated. To rest
on our twenty years experience in vibration calculation the force field
quality makes the main influence on frequencies.

Dr. V.Khavryutchenko
Head of computation spectroscopy group
Kiev, UKRAINE
e-mail:  [email-masked]
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-- 
Emad
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E. Tajkhorshid				
German Cancer Research Center; DKFZ             Tel: +49 6221 42 2340
Dept. Molecular Biophysics (H0200)              FAX: +49 6221 42 2333
P.O.Box 101949			   
69009 Heidelberg, Germany     Email: [email-masked] 
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* "Never express yourself more clearly than you think." -Niels Bohr *
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