Subject: mopac/fortran on different platforms

From: [email-masked]
Date: Tue, 12 Jan 1999 18:19:10 +0100

Dear list members,

I have been working with Mopac93 for quiet a long time and have 
obeserved the following problem on several occasions: Depending on
the platform on which it has been compiled, different results are
obtained when an optimization runs into numerical difficulties.
I have compared the compiled programs which have been generated on
the following systems:
RS/6000 (AIX,xlf) - PentiumII (Linux,g77) - AMD K6/2 (Linux,g77)

In those cases, where the geometry optimization (EF) runs straight
into the energetic minimum, all three versions give nearly the same
energy and geometry.
In cases where the optimization demands a large number of steps,
or when transition states are searched, the three versions tend to
behave much more diverse. To give an example, in one case (TS with
LET DDMIN=0.00) the two Linux binaries give an optimized geometry
while the AIX version stops with a geometry 10 kcal lower in energy
and repeating the warning "NUMERICAL PROBLEMS..".
In another case (reaction path with three points for one dihedral
angle) the AIX version ends without complaint while the Linux
versions give the error message:

        CARTESIAN COORDINATES READ IN, AND CALCULATION 
       TO BE RUN IN INTERNAL COORDINATES, 
       BUT NOT ALL COORDINATES MARKED FOR OPTIMISATION

although the input geometry IS a z-matrix in INTERNAL coordinates
with some parameters marked to be held constant. This can only be
overcome by adding XYZ to the input line, but then the two Linux
version behave different: On the AMD K6/2, the calculation stops after
500 cycles without finding a minimum, on the PentiumII it works, but
gives energies that differ from the AIX-results. This of course might
be the result of the use of cartesian coordinates for optimization.

To conclude this story, I would like to ask the following questions:

1) Has somebody of you already observed a similar behaviour of MOPAC-
versions (or any other program) that have been ported to different plat-
forms, and (if so) 

2) is this due to the different CPU architectures or to the use of
different compilers ?

3) What might be the problem with the internal coordinates that are not
recognized as an internal z-matrix? Is this connected with the format in
which the z-matrix is read in ?

I am looking forward to helpful comments,

sincerely 

Christian Mueck-Lichtenfeld

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Dr. Christian M�ck-Lichtenfeld
Organisch-Chemisches Institut
Westf„lische Wilhelms-Universit„t
Corrensstraáe 40                         [email-masked]
D-48149 M�nster (Germany)             phone +49 251 83-33239
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