Here are some files we use to simulate proteins with a cubane Fe4S4 cluster.
There is a sample leap.in file to illustrate how they are used.

The charges are for the 2- oxidation state (S=0), taken from this paper:

%A J.-M. Mouesca
%A J.L. Chen
%A L. Noodleman
%A D. Bashford
%A D.A. Case
%T Density functional/Poisson-Boltzmann
calculations of redox potentials for iron-sulfur clusters
%J J. Am. Chem. Soc.
%V 116
%P 11898-11914
%D 1994

The other force constants are ones that seem to work pretty well, but they
have not been fully optimized.  So, you should certainly look to see if they
are doing what you want.  You could also use these as a starting point for
your own optimizations.

...regards...dave case

==================================================================
David A. Case                     |  e-mail:      case@scripps.edu
Dept. of Molecular Biology, TPC15 |  fax:          +1-858-784-8896
The Scripps Research Institute    |  phone:        +1-858-784-9768
10550 N. Torrey Pines Rd.         |  skype:                 dacase
La Jolla CA 92037  USA            |    http://www.scripps.edu/case
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