Particle sizes are not constant

Hi all,

I aim to insert 8000 particles with same radius into a 100mm*100*100 cubic box. When I am trying to view the result in paraview, I have selected the 'radius' option from paraview to check the particles size. The result shows that the particles radius is changing throughout the timesteps. Can anyone help with this? Thanks.

# First Trial for model stimulation (real)

# Initialization

# Preliminaries

atom_style granular
atom_modify map array
boundary f f f
newton off
echo both

communicate single vel yes
units si

# Declare domain
region reg block -0.1 0.1 -0.1 0.1 -0.1 0.1 units box
create_box 1 reg

# Neighbor listing
neighbor 0.002 bin
neigh_modify delay 0

# Set up

# Material properties
fix m1 all property/global youngsModulus peratomtype 5.e6
fix m2 all property/global poissonsRatio peratomtype 0.25
fix m3 all property/global coefficientRestitution peratomtypepair 1 0.5
fix m4 all property/global coefficientFriction peratomtypepair 1 0.45
fix m5 all property/global characteristicVelocity scalar 1.

# Define the physics
pair_style gran model hertz tangential history
pair_coeff * *

# Timestep
timestep 0.00001
fix ctg all check/timestep/gran 1000 0.2 0.2

# Gravity
fix gravi all gravity 9.81 vector 0.0 0.0 -1.0

# Walls
fix xwalls1 all wall/gran model hertz tangential history primitive type 1 zplane -0.1
fix xwalls2 all wall/gran model hertz tangential history primitive type 1 zplane +0.1
fix ywalls1 all wall/gran model hertz tangential history primitive type 1 xplane -0.1
fix ywalls2 all wall/gran model hertz tangential history primitive type 1 xplane +0.1
fix zwalls1 all wall/gran model hertz tangential history primitive type 1 yplane -0.1
fix zwalls2 all wall/gran model hertz tangential history primitive type 1 yplane +0.1

# Particle set up
fix pts1 all particletemplate/sphere 15485863 atom_type 1 density constant 2500 radius constant 0.004
fix pdd1 all particledistribution/discrete 15485867 1 pts1 1.0

# Region and insertion
group nve_group region reg

# Particles insertion
fix ins nve_group insert/pack seed 32452843 distributiontemplate pdd1 vel constant 0. 0. -0.5 &
insert_every once overlapcheck no all_in yes particles_in_region 8000 region reg

# Integrator
fix integr nve_group nve/sphere

# Thermo settings
compute rke all erotate/sphere
thermo_style custom step atoms ke c_rke vol
thermo 1000
thermo_modify lost ignore norm no

dump dmp all custom/vtk 1000 post3/cubic_*.vtk id type x y z ix iy iz vx vy vz fx fy fz omegax omegay omegaz radius

# Run
run 25000