Subject: CCL:g03 optimised geometry becomes unoptimised after freq calculation - SUMMARY - Problem not solved

From: Eric Breynaert <eric.breynaert[at]agr.kuleuven.ac.be>
Date: Tue, 14 Sep 2004 11:43:43 +0200

Dear Ccl'ers

On Mon, Sep 06, 2004 at 01:11:35PM +0200, eric breynaert wrote:
 > dear ccl'ers,
 >
 > I'm doing dft (ub3lyp/lanl2dz) calculations on technetium compounds. After
 > a geometry optimisation, i always run a stabe=(opt) and then a freq
 > calculation.
 >
 >          Item               Value     Threshold  Converged?
 >  Maximum Force             .000043      .000450     YES
 >  RMS     Force             .000007      .000300     YES
 >  Maximum Displacement      .000646      .001800     YES
 >  RMS     Displacement      .000162      .001200     YES
 >  Predicted change in Energy=-3.998357D-08
 >  Optimization completed.
 >     -- Stationary point found.
 >
 > Stable reports ->wavefunction  stable under the perturbations considered
 > At the end of the freq calculation the geom optimisation params become
 >
 >          Item               Value     Threshold  Converged?
 >  Maximum Force             .004051      .000450     NO
 >  RMS     Force             .001202      .000300     NO
 >  Maximum Displacement      .126525      .001800     NO
 >  RMS     Displacement      .018760      .001200     NO
 >  Predicted change in Energy=-1.029330D-03
 >
 > Can anyone explain this ? What happens ?

Among others i got the following reply from help[at]gaussian.com

 > Clearly something is different here and I suspect that the SCF converged
 > to a different energy.  When you say that the Stable reported a stable SCF
 > was this STABLE=OPT and perhaps it found an initial instability and then
 > optimized further to find the true ground state?  In this case you would
 > need to reoptimize the structure before running the FREQ calculation.

After checking this, if found that stable did find an instability in the 
wavefunction, so i did a re-optimisation (optimised within 12 steps) and 
then i ran a stable=(opt) calculation again. This time stable again found 
an instability, so again i did a reoptimisation. Only this time i didn't 
reach an optimised structure within 198 steps. When i look at the energy 
changes and the rms gradient i can only conclude that the inital geometry i 
started with after the last stable=(opt) calculation was more optimised 
than all other steps of this last optimisation calculation. This is a link 
to a screenshot of the report gaussview generates. 
http://splash.studentenweb.org/OptPlot.jpg

All comments on this issue are highly appreciated.

Regards,

Eric Breynaert

-- 
Eric Breynaert
Labo voor Colloodchemie
Kasteelpark Arenberg 23
B-3001 Leuven

Tel: +3216321457
Fax: +3216321998