creating dislocations

I need to create a simple screw dislocation in a FCC lattice.Can
anyone suggest how i should go about doing this?
This is what i have tried till now.
I first oriented my FCC lattice along x 1 1 -2 y 1 1 1 z 1 -1 0.Then i
divided my whole box into 4 regions and then used the displace_atoms
ramp mode to shear one of these regions in the new z direction with
the ramp limits from 0 to burgers vector.
This does not work.
Can anyone tell me how to create this.
cheers
Siddharth

Dislocations are complicated geometric entities. If
you understand the geometry, the simplest way to do
this is to write a code/script that generates the atom
positions you want and then input it to LAMMPS as a data file.

Steve