Vibrational Frequencies

[Gary Breton <gbreton &$at$& berry.edu>]

Hello everyone,
 I performed a minimization on a structure using Gaussian 98W
 (B3LYP/6-311+G**) and followed it up with a frequency calculation at the
 same level (of course).  I obtained a single negative frequency at -27 cm-1.
 I then re-optimized the geometry at the same level but this time ignored
 symmetry.  The new E was lower than the previous by 0.03 kcal/mol.  The new
 frequency calculation afforded a single negative frequency at -11 cm-1.
 David Young's "Computational Chemistry" book suggests (page 94) that
 frequency values with a range of about -20 to 20 cm-1 are essentially zero
 within the numerical accuracy of most software packages.  Is this true for
 G98 as well?
 To sum it all up: Can I take a -11 cm-1 frequency to be inconsequential?  or
 do I need to continue to play with the system until absolutely no negative
 frequencies are present?
 Gary W. Breton
 Chair and Associate Professor
 Department of Chemistry
 Berry College
 PO Box 495016
 Mount Berry, GA 30149
 "There's a light at the
 end of the tunnel, but it
 may be an oncoming train"