[lammps-users] Why does my system collapse?

Dear Lammps_users

      I want to equilibrate a nanowire at 300K by NVT ensemble.
      But with my input, my system always collapses,
      even if I change the equilibration temperature to 50K.
      I am puzzled by this problem so much.
      Are there some errors in my input?
      Are there some additional options in NVT for the non-periodic system?
      Any suggestions will be highly appreciated!

      I append my input script at the end of this mail.

-------------Input file----------------
variable x index 30
variable y index 2.5
variable z index 2.5
variable v equal 0.1
variable c equal 2
variable d equal 28

units metal
atom_style atomic

neighbor 2.0 bin
neigh_modify every 1 delay 5 check yes

# Creat geometry

dimension 3
boundary s s s

lattice fcc 3.52

region box block 0 $x 0 $y 0 $z
create_box 1 box
create_atoms 1 box

# Creat group
region left block 0 $c 0 $y 0 $z
group fixed region left
region midd block $c $d 0 $y 0 $z
group belt region midd
region right block $d $x 0 $y 0 $z
group pull region right
group boundary union fixed pull

# EAM potentials

pair_style eam
pair_coeff 1 1 niu3

# Initial velocites

temperature wendu all full
velocity all create 300.0 887723 temp wendu

# Fixes

fix 1 belt nvt 300.0 300.0 0.1 drag 0.2
fix 2 boundary setforce 0.0 0.0 0.0

# Equilibrate (20 ps)

minimize 1.0e-4 100 1000

dump 1 all atom 50 eq_100_10.xyz
dump_modify 1 header xyz

thermo 50
thermo_modify temp wendu

timestep 0.001
run 20000

Best Regards

Zhenyu Yang
  State Key Laboratory of Nonlinear Mechanics (LNM)
  Institute of Mechanics, Chinese Academy of Sciences
  No.15 Beisihuanxi Road
  Beijing 100080, P. R. China
  FAX: 010-62579511
[email protected]...

What do you mean by "collapse" ?