Suppose if one creates a dislocation in a system with two surfaces top and bottom on z axis and periodic in x and y axes. Motions of atoms at the top and bottom layer should be limited along x and y direction and not to move z direction. One can achieve this constraint in LAMMPS by writing as follows, :
region top block 0.0 300.0 0.0 20.0 212.643446964 300.0
region bottom block 0.0 300.0 0.0 20.0 0.0 7.0
group topatoms region top
group botatoms region bottom
fix constraint topatoms setforce NULL NULL 0.0
fix constraint botatoms setforce NULL NULL 0.0
velocity all set 0.0 0.0 0.0
Here two regions are defined, called top and bottom. Then groups of
atoms called topatoms and botatoms are defined, and the forces of z
component are reset to 0.