Unexpected behaviour of the pressure profile using fix NPT

My model is composed of three layers: a layer of rigid and static copper, a mobile layer of copper and a layer of gold. the layer of gold is rigid but globally mobile.
I start by minimizing energy. Then I apply a fix npt with a temperature of 300K.
I calculate both the temperature and the pressure in the mobile copper layer.
I applied a pressure on the top plate in a linear manner until reaching 100 bars. However, when plotting the profile, we can see that there are large oscillations of the pressure (note that thermal profile behaves as expected).
Can you help me in understanding this? (LAMMPS (29 Sep 2021 - Update 2))


clear
units 		metal
dimension	3
boundary	p	p	s
atom_style	atomic

# ----------------------- ATOM DEFINITION -----------------------
# fixed boundary (copper)
region		cufixed		block	-50 50 -50 50	0	9 units box
# layer of mobile copper
region 		cumobile 			block 	-50 50 -50 50	9	50 units box
# layer of gold
region 		layerau		block	-50 50 -50 50 50 75 units box
# simulation box:
region		box			block	-75 75 -75 75	-25	100 units box

#----------------------------------------------------------------
create_box	2 box

# copper---------------------------------------------------------
lattice		fcc 	3.6
create_atoms	2 region cufixed
create_atoms	2 region cumobile
# gold-------------------------------------------------------------
lattice		fcc	 	4.08
create_atoms	1	region	layerau

#----------------------------------------------------------------
group	boundarymotion	region	layerau
group	boundaryfixed	region	cufixed
group	mobile region	cumobile

# ------------------------ FORCE FIELDS -----------------------
pair_style	eam/alloy
pair_coeff * * CuAgAu_Zhou04.eam.alloy Au Cu

delete_atoms overlap 0.1 all all

# ------------------------ TEMP CONTROLLERS -------------------

compute	tempmobile	mobile	temp

# ------------------------ Define Settings --------------------

# pressure in mobile copper
compute peratomcu	mobile stress/atom NULL
compute pcu mobile reduce sum c_peratomcu[1] c_peratomcu[2] c_peratomcu[3]
variable	volcu		equal	410
variable pressioncu equal -(c_pcu[3])/(3*v_volcu)

variable gradual_pressure equal ramp(0,0.00013)
variable 	pressioneq 	equal	"-1 * v_gradual_pressure"

# ------------------------ EQUILIBRATE ------------------------

reset_timestep	0
timestep	0.001
dump		3	all custom	2000 	dump.comp.* id type x y z
log			dump.log
thermo	1000
thermo_style	custom	step	temp	pe	lx	ly	lz	press
thermo_modify	temp	tempmobile

#print "*****************************minimisation nrj***********************************"
min_style cg
minimize 1e-15 1e-15 5000 20000

print "*****************************EQUILIBRE 300**************************************"
velocity		mobile	create	300.0	239834	dist	gaussian
velocity		boundaryfixed	set	0.0	0.0	0.0

# ouput pressure in mobile copper :
fix fixpressurecu mobile ave/time 100 5 1000 v_pressioncu file pressioncu.dat
# output temperature in mobile copper:
fix fixtempcu mobile ave/time 100 5 1000 c_tempmobile file tempcu.dat

fix	1	boundaryfixed	setforce	0.0	0.0	0.0
fix	pnulle	boundarymotion	aveforce	0.0	0.0	v_pressioneq
fix		2	mobile	npt	temp	300.0	300.0	0.1	x	0	0	1	y	0	0	1 drag 1.0
fix_modify	2	temp	tempmobile

run	30000
# SIMULATION DONE
print "All done"```

Both, temperature and pressure will fluctuate over time. But for the conditions you selected and the rather incompressible solid material you have, the pressure fluctuations over time will be much larger. The topic of pressure fluctuations has been discussed many, many times. Please search through and study the forum archives to learn more. Also, looking this up in a statistical thermodynamics text book won’t hurt.

Thanks for your quick response. I will check the archives.