Subject: Re: CCL:M:MOPAC vibrations negative for a ground state?

From: [email-masked] (Ross Nobes)
Date: Tue, 9 Jul 1996 13:55:27 +0100 (BST)

Victor M. Rosas Garcia writes:
  > 
  > So I didn't get any imaginary freqs.!
  > 
  > My only guess as to what could be the difference:
  > * Check the revision of MOPAC93 that you are using.  I'm using RELEASE 2.  That
  > could explain the difference between your MOPAC run and mine.

No, I don't think that this is the explanation. If I sketch the molecule
using Cerius2, snap to Cs symmetry and then run the optimization+
frequency calculation using MOPAC93 Release 1 on an SGI Indy, I also 
get all real frequencies:

   Root No.    1       2       3       4       5       6       7       8

              1 A"    2 A"    1 A'    3 A"    4 A"    2 A'    3 A'    5 A"

              30.8    87.1   151.4   167.1   290.4   309.9   403.4   465.2
I believe that Victor's alternative explanation, namely that the
original structure is a saddle point for rotation of the amino or nitro
groups (or possibly inversion of the amino group), is more plausible.

By the way, the message "SYSTEM IS A GROUND STATE" is a red herring in
thermochemistry calculations. One will always generate this message
(even if there are one or more imaginary frequencies in the force
constant calculation) unless the keyword TRANS has been used to signify 
that this is a transition state.

Best wishes,
Ross
-- 
+----------------------------------------------------------------+
| Dr Ross Nobes                                                  |
| Senior Scientist, Quantum Mechanics                            !
|                                                                |
| Molecular Simulations                                          |
| 240/250 The Quorum                 Phone:  + 44 1223 413300    |
| Barnwell Road                      Fax:    + 44 1223 413301    |
| Cambridge CB5 8RE, UK              E-mail: [email-masked] |
+----------------------------------------------------------------+