Subject: solvation in MOPAC93 using COSMO

From: <[email-masked]>
Date: Wed, 26 Jul 1995 16:26:17 -0400

Netters:

On Jul 26,  8:50am, Victor M. Rosas Garcia wrote:
> Subject: CCL:Failed minimization in MOPAC93
> Hi CCL'rs,
> 	I'm currently minimizing some molecules in MOPAC93, using a dielectric
> constant of 78.3.  Mi starting point is the structure optimized in gas phase
> using MOPAC 6.0.  After the run is finished I get the message:
>
> "THE LINE MINIMIZATION FAILED TWICE IN A ROW.  TAKE CARE!"
>
> And the GRADIENT NORM varies between 2 and 25 (a little too high, I think).
>
> This are the keywords I'm using:
>
> AM1 EPS=78.3 NOINTER T=3600.00 PRECISE
>
> I have looked up in the list of error messages provided in the MOPAC 93
manual
> but the message is not there.  Is this something to be concerned about?
 Should
> I be concerned about the gradient norm?
>
> Any help will be appreciated.
>

Observations:

Using the solvent model, COSMO, in Mopac93 is requires a bit of adjustment in
thinking.  After hundreds of runs using it I have found that the gradient does
not settle down nearly as fast st the heat of formation.  Thus, I have modified
my code to stop after the average deviation value of the heat of formation over
the last 5  geometry cycles is less than 0.01 Kcal. Usually I find that further
grinding gains nothing but lost computer time.  Further I always use the EF
optimiser.

Lastly, I have modified the program to test the sign of the input dielectric
constant; if it is negative then a gas phase optimisation is run first, and
then the solvated run is begun using the gas phase geometry, gas phase Hessian,
and the gas phase density to start the solvated optimisation.  This trouble
helps tremendously.  The optimisation gets as far as it is going to go quicker.

Hope this helps.

Regards,

John McKelvey

-- 

John M. McKelvey			email: [email-masked]
Computational Science Laboratory	phone: (716) 477-3335
2nd Floor, Bldg 83, RL
Eastman Kodak Company			
Rochester, NY 14650-2216

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