Subject: Frequencies calculated at different levels of theory

From: "Emadeddin Tajkhorshid" <[email-masked]>
Date: Thu, 4 Jun 1998 16:50:37 -0700

Dear Colleagues

I have a question regarding the frequencies calculated at different levels of
theory for the same molcule. Can we decide on the basis of the closeness of the
frequencies obtaiened at one level to judge about the accuracy of the
geometrical parameters calculated in that model? In the other words, do the
numbers calculated for the frequencies depend only on the geometry, or there
are other things which may influence the shape of the potential surface around
the minimum and which may significantly differ from a level to another one.

For example, we have calculated the geometry and vibrational frequencies of
molecule A using two models, M1 and M2. The bond distances obtained from M1 and
M2 are different. Can we compare the calculated frequencies to the experimental
ones and say: because the vibrational frequencies of method M1 are closer to
the experimental values, the geometrical parameters (in this case bond
distances) are also more accurately predicted by the method.

Thanx in advance

-- 
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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