[gmx-users] how to restrain four groups!

[nanyu101 at sina.com]

Dear gmx-users,
 
  I must restrain the positions of four groups. Would you please give me some advices?
 
  I have already done the followings.

In my topology file:
include forcefield parameters
#include "ffG43a2.itp"
#include "dpp.itp"
#include "posre_dpp.itp"
#include "phb.itp"
#include "posre_phb.itp"
#include "ions.itp"
#include "posre_ca.itp"
#include "posre_cl.itp


#ifdef POSRES
#include "posre.itp"
#endif

#ifdef FLEX_SPC
#include "flexspc.itp"
#else
#include "spc.itp"
#endif

I just want to restrain the positions of the dpp,phb, ca and cl groups. the ca and cl groups are the ions. How can I edit the posre_ca/cl.itp for the ion groups because the ions have no bonds between the atoms..SO how can i fix this problem?

Many thanks.

Best wishes,
nanyu
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