[lammps-users] The problem of fix addforce and units lj

Hello:
There is a problem in my simulation .I want to control a tip which has 5618 atoms move on a substrate by fix addforce 0 0 -0.01
command. the force of tip should be -56.18 in theory. when the units in lj,the force i apply to the tip output is -0.0004867525.
when i change the units to units metal or units real ,the force of tip is -56.18. is it the command units lj lead to the problem of
the tip force ? here is my in script and datafile:

3d contact simulation

units lj
boundary p p s
atom_style atomic
neighbor 0.3 bin
neigh_modify delay 5

create geometry

read_data yatou.data
lattice fcc 1
region sample block 0 30 0 30 0 30
create_atoms 2 region sample

mass 1 1
mass 2 1
mass 3 1
mass 4 1
#read_restart restart.500000

lj potentials

pair_style lj/cut 1.5
pair_coeff * * 1.0 1.0 1.12
#pair_coeff 1 4 1.0 1.0 1.12
#neigh_modify exclude type 1 1
#neigh_modify exclude type 2 2

region lower block INF INF INF INF INF 2
region newton block INF INF INF INF 10 30
group tip type 1
group sample region sample
group lower region lower
group newton region newton
group mobile subtract sample lower
group constant subtract sample lower newton
set group lower type 2
set group constant type 3
set group newton type 4

temp controllers

compute new mobile temp
velocity mobile create 0.0001 3618360 temp new

timestep 0.005

equilibrate

fix 2 constant langevin 0.0001 0.0001 0.1 3618360
fix 3 lower setforce 0 0 0
fix 6 tip addforce -0.01 0.01 -0.01
fix 4 tip rigid single
fix 1 mobile nve
fix 5 tip setforce NULL NULL NULL

dump 1 all atom 5000 dump.lammpstrj
dump_modify 1 scale no
thermo 100
thermo_style custom step pe etotal c_new f_5[1] f_5[2] f_5[3] vol
restart 500000 restart

run 2000

next is the the datafile yatou.data

5618 atoms
4 atom types

0 47.622 xlo xhi
0 47.622 ylo yhi
0 58 zlo zhi

Atoms
1 1 23.8110 21.4299 48.7420
……
……

When you use units LJ, the default for thermo
output is to normalize by the number of atoms.
If you don't want this then you can use
thermo_modify norm no. The default for metal
and real units is no.

Steve

2010/5/25 Hebing Qiu <[email protected]>: