1D chain of atoms

Dear LAMMPS Users

I am doing a simple 1D chain of atom with lennard jones potential. I am gonna find displacement after relaxation. In data file, changing simulation box in X-direction changes output. How can I eliminate this effect?

Initialization

units real
boundary p p p
atom_style atomic

read_data data.SCB

group clamp id 1
group load id 31

pair_style lj/cut 10
pair_coeff * * 1.0 1.0
################# compute ####################################
compute dis all displace/atom
compute st all stress/atom
################# Equilibration ####################################
fix 1 all nvt temp 300.0 300.0 0.1
fix 2 clamp setforce 0.0 0.0 0.0

thermo_style custom step temp press pe ke etotal lx ly lz
thermo 10
timestep 0.0000010
#-----------------Dump output----------------------------------------------
dump 2 all custom 10000 dump.wire.* id type xs ys zs c_dis[1]
#-----------------------minimization---------------------------------------
min_style cg
minimize 0.00 0 500000000 500000000

SCB model for 1D chain of atoms

31 atoms

1 atom types

0.0000 34.711 xlo xhi
0.0000 10.0000 ylo yhi
0.0000 10.0000 zlo zhi

Masses

1 1

Atoms

1 1 0 5 5
2 1 1.11961086631123 5 5
3 1 2.23922173262245 5 5
4 1 3.35883259893368 5 5
5 1 4.47844346524490 5 5
6 1 5.59805433155613 5 5
7 1 6.71766519786735 5 5
8 1 7.83727606417858 5 5
9 1 8.95688693048980 5 5
10 1 10.0764977968010 5 5
11 1 11.1961086631123 5 5
12 1 12.3157195294235 5 5
13 1 13.4353303957347 5 5
14 1 14.5549412620459 5 5
15 1 15.6745521283572 5 5
16 1 16.7941629946684 5 5
17 1 17.9137738609796 5 5
18 1 19.0333847272908 5 5
19 1 20.1529955936021 5 5
20 1 21.2726064599133 5 5
21 1 22.3922173262245 5 5
22 1 23.5118281925357 5 5
23 1 24.6314390588470 5 5
24 1 25.7510499251582 5 5
25 1 26.8706607914694 5 5
26 1 27.9902716577806 5 5
27 1 29.1098825240919 5 5
28 1 30.2294933904031 5 5
29 1 31.3491042567143 5 5
30 1 32.4687151230255 5 5
31 1 33.5883259893368 5 5

Dear LAMMPS Users

I am doing a simple 1D chain of atom with lennard jones potential. I am
gonna find displacement after relaxation. In data file, changing simulation
box in X-direction changes output. How can I eliminate this effect?

you use periodic boundaries (and very strange LJ parameters, too),
thus your chain interacts with its periodic images. use fixed
boundaries and this will not happen.