Subject: Convergence in MOPAC C.I. calculations

From: <[email-masked]>
Date: Mon, 31 Oct 94 08:24 CST

Hello,

I am using MOPAC 6.0 to do calculations on fairly large polycyclic
molecules related to porphyrins.  Part of this work involves optimizing
geometries and calculating energies for the cations and anions of these
molecules.  To make these calculations more accurate (hopefully) we are
using the configuration interaction options in MOPAC 6, but have encountered
some problems with convergence.  After reducing the gradient to below 1.0,
the optimization begins to oscillate, and shows no sign of convergence
after running for several hours.  A typical command line is as follows:

AM1 PRECISE CHARGE=-1 C.I.=(4,2) VECTORS NODIIS

We have also tried the eigenvector following converger with similar results.
If anyone has had experience with a similar problem, I would appreciate any
advice that you could provide on this subject.  The manual is a bit brief
on both configuration interaction calculations and convergence problems, so
I thought I might try here.  You can mail me directly and I will summarize
the responses to the list if there is interest.  Thanks in advance....

Kristopher Wise
University of Oklahoma
Department of Chemistry and Biochemistry
[email-masked]

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From: "Matthew E. Harbowy" <[email-masked]>
Date: Mon, 31 Oct 1994 20:47:14 +1900 (EST)

>some problems with convergence.  After reducing the gradient to below 1.0,

It's a great idea, one that i've tried to rationalize myself. It's also
a double correction, since parameterization adjusts the energies to 
include the 'correlation' piece once already, and what that means for
transition states is anyone's guess.

>AM1 PRECISE CHARGE=-1 C.I.=(4,2) VECTORS NODIIS

Indeed, I saw the same thing, and EF only makes it worse. NLLSQ was the
one that did the trick, and occasionally I would use SIGMA as well. Also,
you have to be very careful about picking your ci levels. sometimes (5,2)
is a much better choice due to near degeneracies which don't cause it
to abort with errors, but oscillate instead. 

Also, try dummy atoms, and define your geometries in different ways. A
well chosen zmatrix cured some, not all, of my ills. Part of my problem
turned out to be I was attributing more symmetry to the molecule than was 
true by a bad zmatrix definition. XYZ was right out because I was studying
a cyclization, and the orchestrated nature of the reaction required good
control of the zmatrix. I assume for porphyrin analogs you're using 
symmetry to cut down on the number of dependent variables. what I found was 
the forced symmetry led me to a multiple saddle point, not a minimum or
transition state.

>[email-masked]

matt harbowy
tea chemist, thomas j lipton.
-- 
matt harbowy               |  
[email-masked]        | "I'm the bear that went
[email-masked]  |  over the mountain."
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