a question about tensile deformation with the boundary s s p

Dear all,

I want to simulate the tensile deformation of a nanowire with PBC in Z direction (boundary s s p ). One fixed group in one end and one mobile group in the other end of nanowire are defined. Because of the PBC in Z direction the tensile deformation is not well-simulated. I mean, no elongation and no increase in length is observed in Z direction. I think I should increase the length of nanowire each step throughout the tensile deformation but I do not know exactly how! It would be really kind of you if you help me in detail and tell me what commands are better to use. A part of the input script is following:

region fixed block -6 27 -2 42 -3 8 units box
region mob block -6 27 -2 42 34 44 units box

define groups

group fixed region fixed
group mob region mob

pair_style eam/fs
pair_coeff * * NiTii.FS Ni Ti

initial velocities

velocity all create 2.0 5812775 units box
fix 1 all nvt temp 2.0 2.0 0.01 drag 0.5
dump 1 all atom 1000 dump.indent


run 40000

tensile deformation

fix fixed fixed setforce 0.0 0.0 0.0
velocity fixed set 0.0 0.0 0.0 units box

variable a loop 40
label loop
displace_atoms mob move 0 0 0.1 units box
run 2000
next a
jump in.tensile loop