From owner-chemistry@ccl.net Tue Jun 26 16:53:01 2007
From: "Shobe, David David.Shobe[*]sud-chemie.com" <owner-chemistry.:.server.ccl.net>
To: CCL
Subject: CCL:G: strange optimization
Message-Id: <-34579-070626163943-12317-SOgVjOkDs8RGyORNg3DR6w.:.server.ccl.net>
X-Original-From: "Shobe, David" <David.Shobe\a/sud-chemie.com>
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Date: Tue, 26 Jun 2007 22:39:14 +0200
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Sent to CCL by: "Shobe, David" [David.Shobe{=}sud-chemie.com]
Aleksandra,

During the 'opt' stage of the calculation, an approximate Hessian was
used (the Hessian is a matrix of second derivatives which is used to
calculate the displacements).  A more exact Hessian is calculated in the
'freq' stage, in order to obtain accurate vibrational frequencies and
thermochemical parameters.  When Gaussian recalculates the displacements
using the more exact Hessian, sometimes the displacements are no longer
within the convergence criteria.

You can use the keywords "opt=readfc geom=check" to reoptimize using the
new Hessian.  Whether it's worth doing so depends on how large are the
displacements, how important an accurate molecular geometry is, and how
anal you are. :-)

Regards,
--David Shobe

-----Original Message-----
> From: owner-chemistry() ccl.net [mailto:owner-chemistry() ccl.net] 
Sent: Tuesday, June 26, 2007 10:05 AM
To: Shobe, David
Subject: CCL: strange optimization


Sent to CCL by: "Aleksandra N Rudnitskaya"
[Aleksandra.rudnit001%umb.edu]
Dear all:
  
I faced with the problem while I optimized radicals, and molecules they
appeared first as optimized structures then the program calculated
thermochemistry and then right before the answer block it says structure
is optimized partly. I tried to reoptimized the structure but it did not
work.    

        Item               Value     Threshold  Converged?
 Maximum Force            0.000277     0.000450     YES
 RMS     Force            0.000084     0.000300     YES
 Maximum Displacement     0.000898     0.001800     YES
 RMS     Displacement     0.000457     0.001200     YES
 Predicted change in Energy=-2.303047D-07
 Optimization completed.
    -- Stationary point found.
__________________________________________________
.Thermochemistry
___________________________________________________
 Item               Value     Threshold  Converged?
 Maximum Force            0.000277     0.000450     YES
 RMS     Force            0.000084     0.000300     YES
 Maximum Displacement     0.003152     0.001800     NO 
 RMS     Displacement     0.001714     0.001200     NO

 How should I understand that?

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