CNT fluctuates

Dear Lammps User

I have occurred with a problem.
I have set the ends of the CNT to velocity 0.0 0.0 0.0 and force 0.0 0.0 0.0.
The length is of CNT is along z direction.
I have applied velocity as 0.0 0.01 0.0 to middle of the CNT (i.e. the atoms other than the extreme ends).
After the simulations I found that the middle portion of the atoms moved ( still bonded to each other) to maximum distance and then return to minimum distance (i.e. in opposite directions) and thus the CNT fluctuates.

Please tell me what may be the possible reason of such behavior ( because the middle atoms must continuously move until it breaks)

Thanks!!!

What command are you using to move the atoms in the middle?

Are you using the "fix move" command or the "velocity" command?
(I think you should use "fix move", but it sounds like you used "velocity".)
http://lammps.sandia.gov/doc/fix_move.html
http://lammps.sandia.gov/doc/velocity.html
It seems like "velocity" only changes the velocity of the atoms
initially (for example at t=0). But "fix move" maintains constant
velocity (I think). So try "fix move" and see if it helps.

Cheers!

Andrew

P.S.
Once you get that working, I suggest not to move all the atoms between
the ends of the nanotube. I suspect you will have prettier results if
you only move a small number of atoms right in the middle of the
nanotube. You probably would have figured that out. Forgive me if
you did not need this suggestion.

Thanks Andrew Sir

I want to add here that it worked perfect when I used the same conditions(i.e. force and velocity at extreme ends were same ) and applied velocity as 0.0 0.0 0.01 (as strain) and the tube stretch.

What may be the reason here?

Thanks Andrew Sir

I want to add here that it worked perfect when I used the same
conditions(i.e. force and velocity at extreme ends were same ) and applied
velocity as 0.0 0.0 0.01 (as strain) and the tube stretch.

What may be the reason here?

I have to leave very soon so I can't give you a long explanation. The
"fix move" command keeps the velocity of the atoms the same (at all t,
not just when t=0). The "velocity" command only sets the velocity of
the atoms at t=0 (I think). (If I'm right then) this means that the
velocity of these atoms will change as forces are applied to them. As
the center atoms move farther, these forces will be stronger and
eventually they will pull these atoms back to the middle. (It's just
like simulating a pendulum which is already at equilibrium, but giving
it an initial velocity.) I have to run now. I'm glad that worked.

Andrew