question about velocity command


I am trying to perform an uniaxial test of a copper nanowire.
As I have read in some emails before I can’t use fix deform because my system hasn’t got periodic boundary conditions.
I am using velocity command in the upper and lower layers of atoms but when I run the simulation the values of lx, ly and lz are always the same, so it doesn’t move at all.
Do I need to set the force to 0.0? If not, have you got any other suggestion to perform this`kind of simulation?

and the second question, what information do I need to compute to find the Young Modulus?

here is my input script,

units metal
dimension 3
boundary s s p
atom_style atomic
variable latconst equal 3.615
lattice fcc {latconst} region nanowire block 0 20 0 20 0 6 create_box 3 nanowire lattice fcc {latconst} orient x 1 0 0 orient y 0 1 0 orient z 0 0 1
create_atoms 1 region nanowire

pair_style eam
pair_coeff * * Cu_u3.eam

neigh_modify delay 0 every 1 check yes page 100000 one 2000

region upper block INF INF 59.7 INF INF INF units box
group upper region upper
region lower block INF INF INF 12.66 INF INF units box
group lower region lower
group boundary union lower upper
group middle subtract all boundary

set group lower type 2
set group upper type 3

compute centrosym all centro/atom fcc
compute ss all stress/atom virial
compute new middle temp

velocity middle create 300 12345 temp new
velocity upper set 0.0 +0.01 0.0
velocity lower set 0.0 -0.01 0.0

fix 1 all nvt temp 0.01 0.01 1
fix 4a lower setforce 0.0 0.0 0.0
fix 4b upper setforce 0.0 0.0 0.0

timestep 0.01
thermo 500
thermo_style custom step lx ly lz press pxx pyy pzz pe temp

run 1000000

Thank you,


I would worry more about the coords of the
atoms rather than lx,ly,lz which are just the
shrink-wrapped box size. You can dump
atom coords and see if the ones you want are
moving with the velocity you want. Setting
the velocity to a value and the force to zero,
and integrating them with fix nve should do
the trick. You should also viz your system
to see if it's doing what you expect.