[lammps-users] pressure can't be fixed

Hi,

I’m using fix NPT command for argon in lj unit :

fix 1 all npt temp 0.008347 2.4875 10 iso 0.002418 0.002418 1.0
run 1000000

After simulation, I plotted a graph of pressure/Pa vs temperature/K as attached in this mail.
I found out the pressure below ~143K fluctuates widely, while the pressure fluctuates narrowly on the other hand as can be seen from the attached graph. Can anyone explain this phenomena? I wonder why the pressure can’t be fixed well below ~143K.

Thank you.

Christopher

pvst.eps (163 KB)

I don't know, but your damping constants are not ideal.
In LJ units I would use something like 1 for temperature and
2-10 for pressure. So I would try playing around with those
for a small system.

Steve