Hi phenixbb, 

I've got a question related to refinement of an ion on a special position. I have a structure with an atom placed squarely on the C2 axis. I've been able to get a mostly-reasonable model by dropping in the ion at 0.5 occupancy and turning off repulsion for the ion in question, as Pavel describes here. It almost completely works.

The thing I find now is that during refinement there can still be some random drift of the atom away from the symmetry axis, splitting from its symmetry ghost, while I know that it should stay perfectly coincident. My resolution is 3.1 and there's no indication of a multiple conformation, so a split isn't justified. The most parsimonious model should place the ion exactly on the C2. 

Is there any way to enforce that either 1. x, y = 0.0 in refinement, or 2. the ion is constrained to the xyz of the symmetric version of itself. Both should get to the same place, with the metal precisely on the axis, and precisely over top of it's symmetric counterpart. 

Thanks!