Subject: SCF-Convergence problems

From: Mr Andrew D Allen <[email-masked]>
Date: Fri, 15 Apr 94 12:11:21 BST

My original request detailed problems obtaining an SCF for a partially optimised
system. The replys I have got are detailed below.

My original Plea for help...
I am running AMPAC v2.1 and have run into problems with SCF convergence. I have
a large, ~60 atoms, molecule which converges OK if carry out a geometry
optimisation on all coordinates. The problem arises when I try to constrain the
optimisation by flaging some of the bonds, angle and torsions to not be
optimised.

        Has anyone experienced this problem with Semi-emp, or QM methods in
general, and has anyone any possible ways to overcome the problem. I would like
to avoid doing a full geometry optimisation for the molecule due to its size.

Andy.

REPLIES.
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Hi, Andy,

I would appreciate if you let me know (or summarize to CCL) the
answers you get. I started to work on methods to improve convergence
in various contexts. I am not familiar with AMPAC, unfortunately.
Perhaps you could send me some more details?

Best regards

Herbert

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Greetings...Most cases of difficulty I've run into in MOPAC were cured by level
shifting...doe AMPAC do this as well ?

John McKelvey
Wastman Kodak

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Does your AMPAC have the "EF" optimiser ?   If so and it is like the one o
in MOPAC then you can compute the Hessian at the beginning.  That helps a lot.

If you care to send me your original Z-matrix I could take a shot at it, out of scientific curiosity.

John

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 Dear Professor Allen,
 
   If you could send me trhe data file, I'd be happy to take a look at it 
 and see what the difficulty is.  I'll also run it through AMPAC 5.0 and 
 check what the new program does as well.  Please send both data files 
 and the output file for the run that failed.
 
    Andy Holder

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	For the record, AMPAC ver2.1 has SHIFT, PULAY and CAMP-KING convergers
and neither was sucessful in producing an SCF after 200 iterations. As for EF
I have not come across this method.

	I have passed Andy Holder a copy of my input file and as far as I know
he is still looking into it. I have since managed to optimise the the full
structure, and have a gradient of 2.7 Kcal/Ang, I am further refining the
structure to get the gradient even lower.

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Structural and Computation Chemistry [email-masked] - JANET.
Department of Chemistry____________________________phone_______+44(483)-259591.
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