WHY CORRECTIONS TO CALC FREQS

["E. Lewars" <elewars-0at0-alchemy.chem.utoronto.ca>]

 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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