Subject: Vibrational Frequencies

From: Gary Breton <[email-masked]>
Date: Fri, 11 Jan 2002 13:24:31 -0500

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"