mobility of screw dislocation

Hi all LAMMPS users
i have some problem with mobility of screw dislocation simulation . i created the box containing screw dislocation correctly with orientation X=[111] Y=[-1-12] Z=[1-10] which means the dislocation will move in y direction. but when i apply stress in x direction to box (rendering shear stress to box) the dislocation moves in both x and z direction (such a diagonal way and also box is collapsed). I applied the stress 100Mpa (force is correct) by: # boundary conditions fix 1 upper setforce 0 0 NULL fix 2 lower setforce 0 0 NULL fix 3 upper aveforce ${appliedforce} 0. 0. fix 4 upper rigid group 1 upper can you please help me where i did wrong? how can i apply shear stress (for example 100 MPa) correctly to the box?

Thanks in advance