Problem with the refinement of spinel structures

dear all when we are using the FULLPROF software for the rietveld refinement of  spinel structure, the refinement stops because of errot of singular matrix. As the corresponding  space space group is Fd-3m (space group with inversion center), what should be done to overcome this problem? thanks

model based structure refinement

dear all if the sufficient knowledge about the structure is known(eg. lattice parameters, space group, approximate atomic positions etc), then what must be the stategy to solve similar structures. i m working on soft ferrites which have spinel structureand space group Fd-3m. i have made a lot o