Thanks to all the people who replyed. The answer came from Serguei
Patchkovskii:

> You will probably notice that the *differences* in all fixed
Cartesians
> remain the same during the optimization, i.e. the only thing which
gets
> moved is the centre of mass. The total energy (or any other physical
> property) is, of course, invariant to this transformation. If it is
> essential for your to have fixed atoms at precisely the coordinated
> you specified in the input file, just write a simple awk script which
> will translate the optimized coordinates back to coincide with the
> fixed atoms.

Thanks again.

Matthias

--
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G. Matthias Ullmann
Department of Molecular Biology
The Scripps Research Institute
10550 N. Torrey Pines Rd., TPC-15
La Jolla, CA 92037 USA
Phone:  619-784-8889
Fax:    619-784-8896
e-mail: ullmann_at_scripps.edu
www:    http://www.scripps.edu/~ullmann