GSAS constraints on ammonium within a crystal

[Asel Sartbaeva]

Dear all,

        I am currently working on a refinement of a zeolite framework with some 
ammonium content, using GSAS. Does anyone have an example of how to set the 
molecular constraints on a tetrahedral NH4 molecule, as part of a refinement of 
the crystal structure? I would really appreciate seeing an example of how it's done.

Thanks in advance,
Asel Sartbaeva