[lammps-users] About periodic boundary condition

Dear steve,

I have a question about shearing under periodic boundary condition (PBC) in LAMMPS.

I want to give constant shear force by addforce command in top layer along the direction parallel to PBC. (other sides are free surfaces)

In that case, the movement of atoms located in the top layer were observed to move toward the shear force direction but the problem is that they re-entered at the opposite side of boundary due to PBC.

Thins means that box shape is not changed but remained in orthogonal shape so that shear deformation can not be achieved.

I know that shear deformation can be achieved by displace_box command under triclinic condition but in that case, shear force can not be remained in constant value.

I need to apply constant shear force to the specimen along the PBC condition.

In your example case in LAMMPS (shear), shear direction is free surface not PBC so that constant shear force can be achieved.

But in many cases, constant shear force should be applied to the direction along the PBC. (other MD codes are allowed to do this, but i think LAMMPS doesn’t )

Could you give me the solution to this problem?


Jongbae Jeon

Look at the fix deform command = it will shear a triclinic box continuously.
If you want to use a moving top layer, then you want non periodic BC,
which you can set via the boundary command.