Fixing atom positions in PBC calculation

[icamps]

Hello,

I am running PBC optimization with XTB.

After running XTB, all the positions were optimized, creating a non periodic structure as the atoms in one face of the cell are displaced (don't match) compare with the ones at the opposite face.

I modified the input POSCAR with the flags T T T and F F F for the atomic positions to be optimized and the ones that we want to remain fixed but this didn't work.

Any ideas about how to fix this?

Thanks,

Camps