From chemistry-request@ccl.net Wed Mar  6 09:14:26 1991
Date: Wed, 6 Mar 91 08:15 EST
From: "DOUGLAS A. SMITH" <FAX0236@uoft02.utoledo.edu>
Subject: consistency in semiempirical codes
To: chemistry@ccl.net
Status: RO

Here is some food for thought and fuel for a fire regarding MOPAC 5.0 versus
MOPAC 6.0.

Those of you who read the DECnet news feed will know that Brent Bessler 
'ported' MOPAC 6.0 to several platforms and released the code via an anonymous
ftp site.  We found that his code did NOT reproduce the MOPAC 5.0 output
of the many test jobs available, and posted this to the same newsfeed.  Brent
later told me that he had not run all the test jobs.  A lively discussion
ensued in that forum, with Mike Peterson at U. Toronto telling tales of
major woe when trying to port and test MOPAC 6.0 on his Apollo DN10000.

As a result of 1) the above situation, and 2) the fact that I had recently 
been to the USAFA and gotten a version of 6.0 from Jimmie Stewart, Richard
Counts at QCPE asked me to test his DECstation and VAX versions of MOPAC 6.0
for test job output and to update it where ever Jimmie had made bug fixes,
etc.  We are now doing that and should have the final product done by this
weekend for the VAX, DECstation, and Apollo 68000-based platforms.  This code
will then be certified for release through QCPE.

THE QUESTION.  For an identical set of parameters, should MOPAC 6.0 reproduce
the output on any given job that was generated by MOPAC 5.0?  This of course
would not be the case if there were bug fixes, but what about simply changes
in the code (i.e. designed to make the code more efficient)?  What about if
a different optimization routine was used (MOPAC 6.0 has eigenvector following
as the primary optimization routine rather than BFGS as in MOPAC 5.0 or 
DFP as in MOPAC 4.0 - yet the change from 4.0 to 5.0 did not change the 
output).  Richard Counts claims, while admitting to being cynical, that since
these are semiempirical methods he would not necessarily expect the results
to match.  But then comes another question.  If the results don't match, which
is (dare I say?) better?  Or, given that we have 2+ years of research results
using MOPAC 5.0, do we have to redo all of that work in order to switch over
to 6.0 and be able to compare results?

"Life's just full of tough choices, isn't it?"  (Ursula the Sea Witch to 
Ariel, Dsiney's The Little Mermaid).

Doug Smith
Assistant Professor of Chemistry
The University of Toledo
Toledo, OH  43606-3390
voice: (419)537-2116  
fax:   (419)537-4033
FAX0236@UOFT02.UTOLEDO.EDU

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