About unwrapping the chain outside the box in periodic boundary condition.

Please reply to the list not to me, thanks.

I am not a polymers person but what you are describing does not make a lot of sense to me. You want the chains to “really” stay in the same box or you just want to “see” them stay in the same box? You have PBCs all around, why would not the chains move across the PBC? What do you mean by “very far” and what is wrong with that? If you want to actually confine your chains inside A box, then you probably should switch to non-PBCs and use reflective walls.