Graphene tensile tests - Rebo cutoff

Yanbiao, I tried using a boundary condition with sss, but I got an error using the fix deform command saying that the boundary condition needed to be periodic. I assume that the graphene would be bulk graphene then. I haven’t looked at the stress yet, but have only been looking at the critical strain from my results so far. How do you extract the stress? Aren’t the principle stresses predefined variables pxx, pyy, and pzz?