Frequencies calculated at different levels of theory



 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: E.Tajkhorshid - at - DKFZ-Heidelberg.de
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 * "Never express yourself more clearly than you think." -Niels Bohr *
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