Dear All,
My Lammps version is 10 March 2021; I have applied fix add force along (+,-) x-direction only in a simple two-dimensional system interacting via LJ potential. The idea is to study systems under stress. After some time, all particles move along the x-direction together. I have also used the fix momentum command to remove the center-of-mass movement, if any.

Is I am something missing, or how to remove the movement of the whole system together?
Thanks

Dear All,
My Lammps version is 10 March 2021; I have applied fix add force along (+,-) x-direction only in a simple two-dimensional system interacting via LJ potential. The idea is to study systems under stress. After some time, all particles move along the x-direction together. I have also used the fix momentum command to remove the center-of-mass movement, if any.

Is I am something missing, or how to remove the movement of the whole system together?

When you add a force you are asking to accelerate that group of atoms. The behavior you describe is consistent with that. You cannot “remove” it except by not applying a force.

To apply a strain you would need to deform the system which will then result in stress.

Dear Prof.,
Can I divide my system into two equal parts along the y-axis such that half part experiences positive force value and another half negative force value along the x-axis in order to suppress such movement?
Thanks

This doesn’t make much sense. The added force is completely artificial and non physical.
Why not simply deform the system?

Dear Prof.,
I think fixing deform will be applied to the whole box? But I need to apply stress only to a small region of the whole box.
Thanks

Dear Prof.,
I think fixing deform will be applied to the whole box? But I need to apply stress only to a small region of the whole box.

But that is what makes no sense. how would this be physically possible for a real system or meaningful?
Stress is not an input but rather something that happens in reaction so some other change to the system.
And the entire (mobile) system will react to it. Otherwise you would not be doing physics but computer animation.
See for example the Poiseuille flow example in the examples/flow/in.flow.pois input file.

You have a flow between two walls and then you use fix aveforce to push the one wall towards the other and that will then cause the stress in the flow.