[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
-----------------END-----------------

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]
2006-07-17

What do you mean by "collapse" ?

Steve