A question about optimization

[yongjunwinwin]

I have a few questions in doing structure optimization:
1.When should we constrain the cell and when we should untick the constrain, for example when I build a super cell, in which one carbon is substituted by other atoms or a molecule is absorbed on the surface of the graphene, whether should I constrain the cell or not in optimization.
2.If the case is not bulk material, but nanoribbon, what about the same problem mentioned above?

[zh]

If the chosen supercell is large enough, it is no necessary to optimize the lattice vectors of cell, and only the atomic positions should be optimized.