From elewars@alchemy.chem.utoronto.ca  Mon Dec 19 17:29:38 1994
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Date: Mon, 19 Dec 1994 17:05:28 -0500
From: "E. Lewars" <elewars@alchemy.chem.utoronto.ca>
Message-Id: <199412192205.RAA14603@alchemy.chem.utoronto.ca>
To: chemistry@ccl.net
Subject: WHY CORRECTIONS TO CALC FREQS


Peter Gedeck asked why calc frqs are in error and so must be scaled.  The general belief seems to be that it's because the calculations (which use the 
eigenvalues of a force constant matrix) assume the vibrations are harmonic.
However, it has been claimed that "this straightfoward looking consideration
is wrong..."; see M. Flock and M. Ramek, Int J. Quantum Chem., Quantum
Chem. Symposium 27, 331-341 (1993).  Some other refs to vib freq scaling are
Possible sources of error in empirical scaling...  C. L. Janssen and
H. F. Schaefer, J Chem Phys 1991 95 5128.
MP2 scaling  D. J. DeFrees, J Comp Chem 1985 82 333.
There is also a recent (ca. 1993) paper by Pople et al in Israel J Chem
on scaling MP2 freqs.
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