I’m trying to compute the thermal conductivity of Argon gas at STP using the Green-Kubo method as described in the related documentation page. I used the same sample script for solid Argon given at the end of the page. The temperature was set to 70K. And it computed the thermal conductivity as 0.31 W/mK, very close to the reference value given on the page.

I’ve changed the lattice spacing and the volume of the simulation box accordingly, as well as the temperature. I also increased the total number of steps. As a result, the lammps-calculated properties of the system are as follows:
pressure = 0.96 atm,
density = 1.708 kg/m^3 (at STP , it should be 1,783kg/m^3)
temperature = 273K

As you can see, the system is very close to the standard conditions. However, Green-Kubo computes the average thermal conductivity of Ar as 0.0003795 W/mK, which is two orders of magnitudes lower than the expected value of 0.0165 W/mK.

What do you think about the possible reasons for this discrepancy? What is the problem here? I’ve seen many studies using this method for solids and liquids, but I haven’t encountered such computation for gases. Can we use Green-Kubo for gases? If not, why?

Have you tried changing each of these properties individually? Try one simulation with a different lattice spacing but same values of everything else, one with a different volume but same everything else, etc. Then try changing two things, then three. Eventually, you’ll find the answer as to what change caused the discrepancy.