Hi Pavel and Bryan,

thanks for the info. With it I was able to correctly define the symmetry-related links as per this example:

refinement.geometry_restraints.edits {
  bond {
    action = *add
    atom_selection_1 = name ZN   and chain C and resname ZN and resseq    1
    atom_selection_2 = name  NE2 and chain A and resname HIS and altid A and resseq   39
    symmetry_operation = -x+1,y+1/2,-z+1/2
    distance_ideal = 2.030000
    sigma = 0.050
  }

Unfortunately, this didn't have the desired result.  Instead, the addition of riding hydrogens fixed the problem.

Cheers,

Alex