How to extract force?

Hi, I am simulating a cnt under tension. I’ve been having problems extracting force from my simulation. The force I get is zero. Can anyone help me figure out why I can only get zero?

3d tersoff carbon nanotubes simulation(compress 10-10-capped)

units metal
dimension 3
atom_style atomic
boundary s s s
neighbor 2.0 bin
neigh_modify every 1

create geometry

read_data data.cnt.txt
pair_style airebo 3.0
pair_coeff * * C:\LAMMPS\CH.airebo.txt C

define groups

group lower id <= 40
group upper id >= 1361
group mobile subtract all lower
group middle subtract mobile upper

initial velocities

velocity middle create 0.01 4928459 dist gaussian units box
velocity lower set 0.0 0.0 0.0 units box
velocity upper set 0.0 0.0 0.0 units box

thermo 10


fix 1 all nvt temp 0.01 0.01 100
fix 2 lower setforce 0.0 0.0 0.0
fix 3 upper setforce 0.0 0.0 0.0
fix 4 all store/force


timestep 0.003
compute 2 all pe/atom
compute 3 all reduce sum c_2
compute peratom all stress/atom
compute p all reduce sum c_peratom[3]
variable s equal -c_p/vol
variable avepo equal c_3/atoms
variable orilenz equal lz

thermo_style custom step temp etotal v_avepo v_s lz vol
thermo 100

run 1000
dump 1 all xyz 100
dump 2 upper custom 100 dump.myforceupt2 id type z fz f_4
dump 3 lower custom 100 dump.myforcedownt2 id type z fz f_4

for loop

label forloop
variable i loop 186
displace_atoms lower move 0 0 -0.05 units box
displace_atoms upper move 0 0 0.05 units box
run 1000
print “@@@@ i&stress : $i $s”
next i
jump cnt.lmp forloop

What force is zero? Have you tried dumping
the force on all atoms and seeing if what you
are outputting is consistent with that?