Subject: Mopac: Negative frequencies

From: <[email-masked]>
Date: Thu, 4 Jul 1996 02:02:09 +0200

Sorry to put this up to the list, but our server doesn't seem to know about
DASgroup. . .Earlier today, [email-masked] wrote, querying the meaning of
the small, negative frequencies reported in the force calculation of a ground
state.  My $0.02:

There seems to be some question or perhaps even controversy over the meaning of
the negative frequencies (actually, imaginary roots) in Mopac calculations.  In
the case where one is calculating a _transition state_, one expects to get
exactly one imaginary root.  What one frequently observes, however, is one root
which is large and negative (say -450) and one (or more!) imaginary roots which
are small, along the order of the root you report.  The large, negative root is
"real", i.e. corresponds to the reaction coordinate; the small ones seem to be
spurious.  While some investigators interpret them as "higher order" saddle
points (whatever these may be), Hehre has attributed them to mixing in of roto-
vibrational states (or some such bumpf -- you can tell I'm not a theoretician),
which can be safely ignored.

I expect that you are running into the same phenomenon, but for a mimimum at
which exactly zero imaginary roots should be observed.  As a practical matter,
you can ignore them, or you can try to get rid of them.  To get rid of them, a
reasonably sure, but painful way, is to follow Stewart's recipe for "escaping
from a hilltop" in the Mopac documentation.  Alternatively, you can do the
computational equivalent of "purification by resubmission": simply rerefine the
structure, starting with the coordinates from the .arc file, or (perhaps better)
from something like a Sybyl .mol2 file.  _Don't_ use the restart file of the
previous calculation. . .the rounding and truncation errors introduced in the
other results files is frequently a sufficient perturbation of the structure to
get rid of the nuisance frequency.

sb

===========================================================================
  Stephen B. Bowlus, Ph.D.                Computer-Aided Molecular Design
                                          Research Division
  e-mail: [email-masked]               Sandoz Agro, Inc.
  Phone:  + 1 415 354 3904                975 California Ave.
  Fax:    + 1 415 857 1125                Palo Alto, CA 94304
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