For the self-diffusion of gas molecules in an isotropic medium, the diffusion coefficient can be evaluated from their trajectory by using the Einstein s definition relating the correlation function of atomic positions r to the diffusivity D = lim t ( |r(t) r(0)|2 /2dit, or the Green Kubo relation using the velocity autocorrelation function D = 0 v(0)v(t) dt/di. Here di is the dimension of space, t is the simulation time and is the ensemble average.