hello,

I am trying to relax a small polymer network with about thousands of atoms in LAMMPS to get a correct density. I minimized the energy first and run MD for 20000 steps under NPT ensemble. The problem is that the volume of the box keeps increasing to several times of the original one while temp and pressure fluctuate in a normal range. I wonder if this is normal or i got something wrong. Here is the input file. Thanks!

units real

atom_style molecular

boundary p p p

pair_style lj/cut 10.0

read_data custom.dat

pair_modify mix geometric

pair_coeff 1 1 0.0038 3.195

pair_coeff 2 2 0.02378 3.8983

pair_coeff 3 3 0.02378 3.8983

pair_coeff 4 4 0.01935 3.6621

pair_coeff 5 5 0.02393 3.4046

pair_coeff 6 6 0.02393 3.4046

pair_coeff 7 7 0.0860 4.03

thermo 100

minimize 1.0e-4 1.0e-4 1000 10000

dump 1 all atom 100 custom.dump

fix 2 all npt 300.0 300.0 100 xyz 1.0 1.0 1000.0 drag 2.0

run 20000