Why is pressure increasing of a relaxed system upon inserting a void at one edge?

I intend to do moving piston based nemd shock simulation for an explosive material. For this I am keeping a void portion at one edge. While doing so in NVE ensemble pressure value of the relaxed structure increases. When I do the simulation without void the pressure stays around 0, whereas with around 30-40A void size in X direction, the pressure increases upto 4000-5000 bar. Why is this happening? and How can I get rid of this?

The input script is as follows:

units real
atom_style full
boundary p p p
bond_style harmonic
angle_style harmonic
dihedral_style harmonic
improper_style cvff

read_data trial.dat

pair_style lj/cut/coul/long 9.5
include paircoefficient.txt
kspace_style pppm 1.0e-6
pair_modify mix arithmetic
special_bonds amber

special_bonds lj 0.0 0.0 0.5 coul 0.0 0.0 0.5

neighbor 2 bin
neigh_modify every 10 delay 0 check no

#============= minimize ================================================


timestep 0.1

#velocity all create 300 12345 mom yes rot no
#min_style cg
#min_modify line quadratic
#minimize 1e-40 1e-40 1000000 1000000

#write_data modeltkx50_14.data
#write_restart 50.restart

variable tstp equal 0.1
timestep ${tstp}
variable tdamp equal “v_tstp * 100”

velocity all create 300 12345 mom yes rot no

fix 1 all nve
fix 2 all langevin 300 300 ${tdamp} 90429997


compute ch_id all chunk/atom bin/1d x lower 0.05 units reduced

fix ave_ch_d all ave/chunk 1 10 100 ch_id density/mass file density.out

fix ave_ch_v all ave/chunk 1 10 100 ch_id vx file velocity.out
fix ave_ch_t all ave/chunk 1 10 100 ch_id temp file temperature.out

compute temperature all temp/chunk ch_id temp com yes

fix ave_ch_t2 all ave/time 1 10 100 c_temperature[1] file temperature2.out mode vector

set number of bins

variable nbins index 20
variable fraction equal 1.0/v_nbins

define bins as chunks

compute cchunk all chunk/atom bin/1d x lower ${fraction} units reduced
compute stress all stress/atom temperature

apply conversion to pressure early since we have no variable style for processing chunks

variable press atom -(c_stress[1]+c_stress[2]+c_stress[3])/(3.0vol{fraction}) variable shear atom -(c_stress[4])/(vol*{fraction})
compute binpress all reduce/chunk cchunk sum v_press
compute binshear all reduce/chunk cchunk sum v_shear
fix avg all ave/time 1 10 100 c_binpress mode vector file ave_stress.dat
fix avg_shear all ave/time 1 10 100 c_binshear mode vector file ave_shear.dat

thermo 1000
thermo_style custom step lx ly lz press pxx pyy pzz pe temp pxy pyz pxz density

dump 1.50 all custom 1000 dump1.mixed.lammpstrj id mol type q xu yu zu ix iy iz
run 80000

write_data modeltkx50_20.data
write_restart 50.restart

unfix 2

It is not clear from your post what you are comparing to what.

Please also note that your input is partially misformatted since you did not wrap it in triple backquotes (```). The forum software will interpret certain special characters and formatting keywords. Also, it is rather confusing to include commented out commands or commands that are irrelevant to the problem at hand.

It would be best, if you created two minimal “before” and “after” inputs (they don’t have to be realistic, but just demonstrate the situation quickly and with the minimum number of input commands).