unstable structure at zero pressure

Dear lammps users,

I am trying to use buckingham+coulombic potential to build a zirconia structure. I find that if the atoms fill the whole simulation box, the structure is stable, but I’m trying to simulate nanoparticle so a cluster will just occupy a small region, then the structure always become unstable. Is this because the atoms at cluster boundaries no longer have forces from neighbor as a result of PBC?

Any suggestion is well appreciated!