Elastic Strain

It is a kind of vague description for “there’s no contraction as much as elastic strain?” You need to give more details, from input script to output results.


I want to simulate a tensile test.


  1. “fix deform” command has to be used along with periodic boundary conditions. Yes, if you turn off this by unfix it, the system should correspond to “unloading”.
  2. If you pull a nano-block with velocity method, what you can do for “unloading” is to release the fixed region/group and let the system run. The stored strain energy will drive the system back to stress-free state as you wished.

Hope it helps.


Thank you Alberto

1)As you saw I sent the input file that used velocity command rather than fix deform. I concluded that fix deform doesn’t work for my goal.
I wanted to have LAMMPS users confirmation on my deduction.
2)My problem is releasing the speeding wall. I make them speed with velocity command. I don’t know how to release them.
Due to set velocity for the wall in z-direction I used

velocity Upper set 0.0 0.0 ${upper_vel} units box

And as I said for releasing it from speeding I used

“velocity Upper set 0 0 NULL units box”

but the wall still is having velocity, and the command with NULL didn’t work.
So the main question still lasts:
How can we remove the velocity for a wall after defining it as if we never defined the velocity firsthand?

THANKS again.

Hello Reza,

I know what’s yield strength and understand what you are trying to do, but you need to have a good description about what you have done with LAMMPS, from input script to output analysis and visualization. Two reminder: 1. The “fix deform” is applied with periodic boundary conditions, 2. The “fix deform” works fine with the “fix npt” - just control the pressure in the unstrained directions, so that they can respond to the strain in a natural way, 3. the deformed system may need more time to bounce back. 4. Carefully choose “tdamp” and “pdamp” parameters in the “fix npt” command. Get good advices from the documentation and do tests on small systems first.