Subject: Re: CCL:Converge problem in the optimization on G94

From: <[email-masked]>

Seiji Mori wrote:

   I use the gaussian 92 and g94 programs. I have a problem about the 
   geometry optimization and frequency analysis. After the geometry 
   optimization completed ,I performed the frequency calculation analysis. 
   However, from the output of frequency analysis, sometimes Optimization 
   is not completed because values about displacement exceed the 
   thresholds. How should I resolved it and wolud you tell me your thought 
   about this problem ?

We have also run into this problem with Gaussian, as well as other ab inito 
packages, to which the following comments and suggestions by Doug Fox, 
Gaussian Inc., on this subject are probably applicable:

   There are several things contributing to your confusion [about 
   optimizations converging only to have successive frequency calculations 
   indicated as not completely converged,] most of them not bugs but rather 
   facts of life.
   
   1) The optimization deals with an approximate Hessian which is updated
   at each step by looking at the differences in gradients.  As the point
   in question corresponds to a zero gradient in the majority of cases minor 
   errors in this Hessian are of no consequence.
   
   2) For some cases, especially cases with very flat interaction potentials
   or near zero barriers, when you replace the approximate Hessian with an 
   analytic second derivative the gradient is still converged but the 
   predicted displacement is no longer.  If you restart the optimization with 
   Opt=(READFC) after the Freq it will carry on for a cycle or two and 
   converge.  Then you can assess the changes.  In some cases you can look at 
   the proposed changes before this and see that it is a methyl rotor which 
   has no impact on the chemistry or some such and ignore it.
   
   As to tightening up the convergence criteria, this can be helpful if you
   have seen this behaviour for one or two of a group you are studying and use 
   OPT=TIGHT on the rest.  I would not suggest turning this on as default 
   because it will add several cycles to all optimizations, well behaved or 
   not.
   
   Another thought which may be useful is to use OPT=LOOSE which will get an 
   approximate structure, run an OPT=CALCFC starting from this structure and 
   then run FREQ at the end.  You still have to run two frequencies but the 
   number of steps between the final structure and the CALCFC will be small 
   and fewer errrors will accumulate.
   
   3) The appearance of a second negative (imaginary really) frequency can
   occur for the above situations, a methyl rotor locked into a maximum with 
   insufficient force to get it free, or it can be a symmetry forbidden mode 
   which is tested by the FREQ.  In the former case you can twist the methyl 
   and restart the optimization.  In the latter you should consider running an 
   IRC to trace this mode down to the minimum.  Also you should display
   this mode graphically to see what chemistry is involved, Spartan can do 
   this as can Cerius2 from MSI.
   
   You can distinguish these cases to some extent based on the symmetry of
   the mode.  If the mode is totally symmetric then it is an artifact and 
   restarting the optimization will fix it.  If not you will have to modify 
   the starting structure to break the symmetry to find the true minimum.
   
   Doug Fox
   [email-masked]

My thanks to Doug Fox for his assistance.

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