CCL:G: strange optimization

 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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