Data was collected with a laboratory x-ray source in transmission geometry. I have tried to refine the site occupancy parameter for my sample using GSAS+EXPGUI program.The refinement was done by constraining the two atoms sharing the same site to have a total occupancy 1. I refined the site occupancy along with all other structural parameters. It gives a negative value for one atom and a number greater than one for the other atom. What should I do to solve this?