I am trying to model 3D Poiseuille flow of an argon gas confined between two parallel platinum plates by using LAMMPS. I have one reference paper, as given in  below. I built my platinum structure correctly and I placed Ar atoms and run my simulation. At the end of simulation, I checked the density and it was totally wrong. I suspect that, I am not able to build sufficient number of atoms in my model. I try to use equations given below to obtain number of atoms but I couldn't decide the number of Ar atoms in my model, still. So, I need your help.
In the paper, they obtain the diameter of atoms by using the Knudsen number. After that I divided this into 2 to get the radius, then I obtained the lattice parameter by $ 2 \sqrt2 r $. Then, I build argon as lattice fcc a. However, the final result and density is not correct. Therefore, I suspect that, the number of atoms in my system is wrong.
So, my question is, how I can obtain lattice constant from the Kn number and obtain the correct density, for instance, for the Kn = 0.1 case in the reference paper  ?