From owner-chemistry@ccl.net Tue Apr  5 12:33:00 2011
From: "Pierre Archirel pierre.archirel---u-psud.fr" <owner-chemistry%%server.ccl.net>
To: CCL
Subject: CCL: IRC for flat PES
Message-Id: <-44297-110405122021-17714-wRVe8VpVubvLUsgdRUDTVw%%server.ccl.net>
X-Original-From: "Pierre  Archirel" <pierre.archirel*|*u-psud.fr>
Date: Tue, 5 Apr 2011 12:20:18 -0400


Sent to CCL by: "Pierre  Archirel" [pierre.archirel..u-psud.fr]
This is an answer to E. V. Horbatenko
If the PES is very flat there is a difficult issue in the separation of the center of mass of the molecule. It is very useful to compare the "negative" frequency characterising the TS with the six "zero" frequencies coming from the global translation and rotation. I have two examples of this, for the same molecule, with B3LYP and int=ultrafine:
Example 1:
 Low frequencies ---  -36.0058  -2.8583  -0.0001   0.0005   0.0009   5.9011
 Low frequencies ---   10.3067  18.8761  33.4494
it can be seen that the "TS" frequency (-36.0058) is much larger (by a factor 10) than the first "zero" frequency (-2.8583). Actually the IRC calculation worked nicely.
Example 2:
 Low frequencies ---  -13.7156  -6.3252  -3.9374  -0.0003   0.0003   0.0005
 Low frequencies ---    5.4561  16.2158  28.6961
In this case the "TS" frequency (-13.7156) is only two times larger than the first "zero" frequency (-6.3252). I could not make the IRC calculation even start. Obviously in this case the "TS" direction is polluted by global rotation.
Have you compared these values?
Pierre Archirel
LCP, Universite Paris-Sud, France
pierre.archirel.u-psud.fr