LAMMPS: Why does matter spontaneously cool/heat up?

Question

Physics

There is a script that simulates argon between two copper plates
. Argon and the right plate are set to a temperature of 300K, and the left one is set to 600K.
However, after 0.1 picoseconds (1000 steps), the temperature of the left plate drops to almost 300K, and the right one to 185K Argon
density - 1.78*10^(-3) g/cm3 (i.e. n.o.; 10^5Pa, 273K), and the copper lattice spacing is 3.62Å
Why does the temperature drop sharply?
At the same time, if the density of argon is increased to 0.05 g/cm3, then the temperature, on the contrary, increases sharply, and this is not heat transfer - during this time, argon does not even have time to touch even one plate.

What am I doing wrong?

input file

units real
#vol/fcc=2 for n.c.
variable offf equal 0    #offset
variable sb equal 55     #y length
variable ld equal 55     #x and z length
variable oft equal 7     #width of Cu plates
variable sdc equal 48    #same as oft
variable arro equal 40   #density of Ar
variable curo equal 3.65 #density of Cu

boundary f f f
dimension 3
atom_style atomic

#setup the whole sim-box
#lattice fcc ${arro}
region box block 0 ${ld} 0 ${sb} 0 ${ld} side in units box
create_box 2 box

#right&left - regions of Cu
#lattice fcc ${curo}
region left block 2 53 2 9 2 53 side in units box
region right block 2 53 48 53 2 53 side in units box
#center - region of Ar
region center block 2 53 9 46 2 53 side in units box

lattice fcc ${arro}
create_atoms 1 region center
lattice fcc ${curo}
create_atoms 2 region left
create_atoms 2 region right

group lg region left
group rg region right
group cg region center

mass 1 40   #Ar
mass 2 63.5 #Cu

####################
#This data from
#https://courses.physics.illinois.edu/phys466/sp2013/projects/2001/team5/node13.html
pair_style	lj/cut 3.54
pair_coeff	1 1 0.234 3.41
pair_coeff      2 2 13.238 2.27
pair_coeff      1 2 1.762 2.84

#Computing kinetic energy per each atom
compute kin all ke/atom
compute lgt lg temp
compute rgt rg temp
compute cgt cg temp

#comm_modify cutoff 3.54
reset_timestep 0
timestep 0.1

#Speeds of atoms like it's 1000K
velocity rg create 300 12345 mom yes rot yes
velocity cg create 300 12345 mom yes rot yes
velocity lg create 600 12345 mom yes rot yes

fix 1 all nve
fix 2 all wall/reflect xlo EDGE xhi EDGE
fix 3 all wall/reflect ylo EDGE yhi EDGE
fix 4 all wall/reflect zlo EDGE zhi EDGE

thermo 1000
thermo_style custom step temp c_lgt c_cgt c_rgt

#Dump for OVITO
dump 1 all custom 10 dump.comp.* id type xs ys zs c_kin fx fy fz

run 50000

Answer 1

variable arro equal 40 #density of Ar
variable curo equal 3.65 #density of Cu

where is this from? The density of copper is 8.92 g / cm³, you yourself write:

Density of argon - 1.78*10^(-3) g/cm3

, and in the input file 40.