Atoms escape boundary after relaxation

[Seunghwan_Kwon]

Hello,

I did relaxation of surface slab (a few bottom layers fixed, a few top layers relaxed, and vacuum above the surface)

After that, I put an oxygen atom on top of slab and did relaxation to calculate oxygen adsorption energy.

Then, some atoms escaped the boundary of slab in CONTCAR.

Position of some atoms were negative sign and those were escaped.

I typed 'ISIF=2' in INCAR since this is slab calculation.

Is it right that atoms escape from the slab?

Intuitively, this is unphysical that atoms escape boundaries.

Thank you.

[alexey.tal]

Dear Seunghwan_Kwon,

Because of the periodic boundary conditions the atoms can shift across the boundary and appear on the other size of the cell.
How do you mean that some atoms escaped? Could you provide the files (guidelines)?