I have a serious problem! I am working on a new structure, on X-ray
  powder data, with GSAS and Fullprof, and I would like to constrain 
  my oxygens (8 with x, y, s as variable!!! plus 6 cations for a total
  of 36 parameters only for the position).
  My oxygens are going everywhere, not very stable and I get too many
  short distances (dis<2.5 Angstrom !!). I would like to put constraints
  in my model. In fullprof I am very lost, in GSAS this is the same
  problem, after I enter the distance and its standard deviation
  plus the name of the two atoms, the list option gives me zero constraints
  If someboby can give me some help.
  
 Dr. Francois Goutenoire
 NorthWestern University
 Dept. Chemistry
 2145 Sheridan Road, Evanston, Illinois 60208, US
 
 Phone: (847)-491-4228
 E-Mail: fgoute@nwu.edu