# distribution of the pressure in gas

Dear my friends

Actually, I want to calculate the pressure distribution in gas that is between two parallel plates. Since the “compute pressure command” gives a mean pressure for all system, I have used “compute stress/atom command” along with “compute chunk/atom command” so the pressure in each bin is calculated.

In the input file, I have written like this for calculating the pressure:

compute p flow stress/atom NULL

then

fix 1 flow ave/chunk 1 10000 10000 cc1 c_p[1] c_p[2] c_p[3] c_p[4] c_p[5] c_p[6] file Pressure.txt ave running

Finally, I got this as the output: (for brevity, only for one the bins is written here)

6 29.7 42.449 -40164.7 -40738.9 -40770.7 -32.0077 28.5133 -5.2131

Actually, I want to calculate the pressure based on that, how can I do it?

In fact, in the LAMMPS User’s Manual, there is one example that calculates the pressure for the whole volume not for each bin. How can I calculate the pressure for each bin?

Actually, my LAMMPS calculation was based on real unit and I just want to know how to convert it to the SI unit?

I would be really grateful if anybody can help me

Best Regards,
Reza

The compute stress/atom command explains how to compare
its results to the total system pressure - the results should
be the same. If you want “pressure in some local region”
you have to normalize by a volume, either per-atom or per-bin,
as the compute stress/atom doc page explains.

A text book (or google) can explain how to convert
P in atmospheres (real units) to any other units.

Steve