Thermal conductivity of dilute argon gas

Hello all,
I am trying to calculate the thermal conductivity of dilute argon gas using green kubo function. After running the lammps simulation I got a value of 0.0003 W/m.k for thermal conductivity. I tried a large simulation box sized 5200 angstrom cube with 52 unit lattice spacing. The temperature was 298 k. Why am I not getting the perfect thermal conductivity? What should I do?

This very same question has been asked many times on this forum, so I would start by looking at the answers that were given at the time. You should also have a look at the forum guidelines as your post violates almost all the rules.