regarding GCMC simulations

hi all,

i want to reproduce work GCMC part of paper of H2  adsorption in MOF-5. energies are calculated by UFF force-field. they  modeled  H2 molecule by placing a ghost atom with charge -0.966e at the  center of the H2 molecule, and charge +0.483e on each H. for H2   molecules. The force field was modified, a repulsive potential   U=D0exp[y(1-R/R0)]  was used .R=inter atomic distance. how to assign that  central negative charge on H2 (should i make 3 body system?) and how to modify repulsive potential in UFF in Accelrys? reference paper is attached with this

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