From chemistry-request@ccl.net Thu Apr 16 15:20:22 1992
Date: Thu, 16 Apr 92 13:18:51 EST
From: Jack Houser <R1JJH%AKRONVM@VM1.CC.UAKRON.EDU>
Subject: PAHs in MOPAC
To: Supercomputer bulletin board <chemistry@ccl.net>
Status: R

Dear Netters:

I'm having a problem getting large PAHs (coronene, alkyl pyrenes, etc.) to run
in MOPAC5 (AM1) on an IBM 3090.  The initial Z matrix is produced using PC-
Model and is checked by hand before running.  The problem is that I get
either a message saying SMALL CHANGES IN INTERNAL COORDINATES ARE CAUSING
A LARGE CHANGE IN THE DISTANCE BETWEEN CHEMICALLY BOUND ATOMS.  THE OPTIMIZ-
ATION WOULD LIKELY PRODUCE INCORRECT RESULTS or, if I introduce some symmetry
to reduce the number of variables, the optimization starts, but then I get
a "fixed point exception" error, which indicates that a calculated number is
too big for the computer's word length.  Has anybody else run into this
problem?  Any suggestions?

Thanks,

Jack Houser
Department of Chemistry
The University of Akron
Akron, OH  44325-3601