Strange pressure with Lennard Jones system

LAMMPS LAMMPS Mailing List Mirror
1

Dear Lammps users, I have a question about the "melt" example of LAMMPS; in particular, I am confused about the pressure output values.
I am running the example input script (inside "share/lammps/examples/melt"), and I am adding the tail corrections too (via pair_modify tail yes)... now, the program generates pressure values P* around 6, which correspond to more or less 2500 atmospheres for Argon... and this value seems to me to be totally non-sense!
The temperature is a much more reasonable value of T* = 1.5 (180 K for Argon). The simulation is run at a reduced density of 0.8442.

What is wrong with that? What information am I missing?
The only other parameter I have changed, with respect to the original input script, is the total number of steps (from 250 to 10000).

Thanks in advance and greetings for the New Year,

Francesco Mambretti

2

Please note that the melt example is just that: an example. It’s just some starting configuration that runs for a few time steps. If you want to obtain thermodynamic properties you’ll need to properly equilibrate the system at the desired conditions.

Kristof

3

I’ll add that a T* = 1.5, which is high, it’s likely
that a large component of the pressure is kinetic.
You can use the compute pressure command with
various keywords to see which components are
contributing to the 6.0 value.

Steve