Simulating Argon gas enclosed by copper walls in lammps

LAMMPS LAMMPS Beginners
1

I am trying to simulate a system of argon gas enclosed by copper walls. I want to equilibrate the system at 300k. However, my gas temperature was reduced to an average of 221k in the results.
My script is given below. (I am new to Molecular Dynamics).

#…
units metal
dimension 3
boundary p p f
atom_style atomic

region box block 0 50 0 50 0 100 units box
create_box 2 box

lattice fcc 3.615

region copper_bottom block 0 50 0 50 0 14.46 units box
create_atoms 1 region copper_bottom

region copper_top block 0 50 0 50 85.54 100 units box
create_atoms 1 region copper_top

region gas block 0 50 0 50 14.46 85.54 units box
create_atoms 2 random 500 12345 gas

group gas type 2

mass 1 63.546
mass 2 39.948

neighbor 0.1 bin
neigh_modify delay 10 check no

pair_style hybrid eam lj/cut 10.0
pair_coeff 1 1 eam Cu_u3.eam
pair_coeff 2 2 lj/cut 0.0104 3.4
pair_coeff 1 2 lj/cut 0.065014 2.8

region fixed_bottom_layer block 0 50 0 50 0 3.615 units box
region fixed_top_layer block 0 50 0 50 96.385 100 units box

group copper_fixed_bottom region fixed_bottom_layer
group copper_fixed_top region fixed_top_layer

fix freeze_bottom copper_fixed_bottom setforce 0.0 0.0 0.0
fix freeze_top copper_fixed_top setforce 0.0 0.0 0.0

region copper_vibrating_bottom block 0 50 0 50 2 14.46 units box
region copper_vibrating_top block 0 50 0 50 85.54 96.385 units box

group copper_vibrating_bottom region copper_vibrating_bottom
group copper_vibrating_top region copper_vibrating_top

group copper_vibrating union copper_vibrating_bottom copper_vibrating_top

fix 2 copper_vibrating nvt temp 300.0 300.0 0.1

velocity gas create 300.0 12345

fix 1 gas nvt temp 300.0 300.0 0.1

compute cc18 gas chunk/atom bin/1d z lower 0.65376 bound z 14.46 85.54
fix f18 gas ave/chunk 1 10000 10000 cc18 density/number temp fx fy fz ave running file denargon_1(gas).txt
compute cc19 gas chunk/atom bin/1d z lower 0.65376 bound z 14.46 85.54
fix f19 gas ave/chunk 1 10000 10000 cc19 density/mass temp fx fy fz ave running file denargon(gas).txt

thermo 50
thermo_style custom step temp press etotal

minimize 1.0e-4 1.0e-6 1000 10000

dump 3 all custom 10000 dump_nvt_*.cfg id type x y z

timestep 0.005
run 1000000

2

Where are you measuring the fluid temperature? From what I see you are outputting the entire system temperature, not the fluid temperature (edit: since you have frozen atoms with temperature 0K, then the temperature of the entire system is expected to be lower).

Then take a look at the rules of the forum for your future posts.

3

Thank you very much.