Large squeezes and crinkles of the atomic structure under equilibrium

I simulated the relaxation process of a double-layer graphene (30nmX10nm) under NVT.
During the simulation, the long edges appear to have been squeezed toward the center and
kept shrinking. The log file shows that the temperature has been stable at 300K, but the
pressure gradually increases negatively to over 1000 before stabilizing. The box is large enough,
so it’s probably not because the interatomic forces are too strong. Then I tried to change the
boundary condition and the temperature, but it didn’t work. I guess the problem comes
from the interaction potential between layers ( kolmogorov/crespi/full ). However, this interaction
potential is carefully designed to describe the long-term interactions between graphene layers.
And I’ve used this potential before. Finally,I really want to get your suggestions.

Here is my script,

units metal
dimension 3
boundary f f f
atom_style bond
neighbor 3.0 bin

pair_style hybrid/overlay rebo kolmogorov/crespi/full 10.0 1
pair_coeff * * rebo CH.airebo C C
pair_coeff 1 2 none
pair_coeff 1 2 kolmogorov/crespi/full CH.KC C C

mass 1 12.01
mass 2 12.01

#define boundary, left and right.
region left block -150 -145 INF INF INF INF
group left region left
region right block 145 150 INF INF INF INF
group right region right
group boundary union left right

velocity boundary set 0.0 0.0 0.0
fix 1 boundary setforce 0.0 0.0 0.0
velocity all create 300 123456

fix A1 all nvt temp 300 300 0.02
timestep 0.0002

thermo_style custom step temp press
thermo 1000
dump X1 all custom 1000 X1.dump id type x y z
run 40000

please a) upgrade to the current version of LAMMPS you seem to be using an older version and you may be missing bugfixes, and b) have a look at the example for this kind of system in the examples/USER/misc/kolmogorov_crespi_full folder of the current LAMMPS distribution. there are some significant differences.