Cacu metal chelating clusters,use what software

 Dear CCLs:
  I want to caculat the energy of thiol-metal chelating
 cluster.It include six CH3CH2S and two Zn++ or Cd++
 formed two metal chelating cluster,I want to caculate
 the change of potential of this structure  with regard
 to the distance between sulphur atom to the metal ion
 and the distance between the two metal ions,can you
 suggest me what kinds of method and what software
 is most perfect,a shared software is best.
 Thank you.
                     sincerely yours
                     Tsinghua University