Dear Surender, you have to frame your question better. What do you want to simulate? It's not clear what is your objective. 

thanks sir,what I mean is that when we were simulating cnt and specially in case of single walled nanotube ,there we cant move beyond 1nm app diameter,is there any such constraint in case of graphene nano ribbons also or not sir.

Graphene is the mother of all forms of nanocarbon. It can be rolled in Bucky balls, nanotubes and stacked into layers of graphite. So for graphene nano ribbon your limit should be 1 nm as it will become a nanotube when rolled with specific chirality.

Balaji sir,this thing i know and so i have written also that when we'll fold the graphene sheet ,so i have written for SWCNT(single walled carbon nanotube)when diameter was approxiamtely 1nm,but what i am asking is that whether there is any limitation on width also in case of graphene nano ribbons also or not sir?

regards