Self-diffusion coefficient of a simple liquid in the subcritical regime of temperature

We present an improved version of the recently introduced formulas for the self-diffusion coefficient in the modified free volume theory of diffusion in simple fluids of particles interacting with Lennard-Jones potential. A suitable modification of the soft sphere levitation diameter and the replacement of the hard sphere radius with the Chapman-Enskog collision bracket integral in the pre-exponential factor improves the accuracy of the formulas proposed by Laghaei et al. [J. Chem. Phys. 124, 154502 (2006)] and Qin et al. [J. Phys. Chem. B 113, 4751 (2009)] and extends their range into both low density and liquid regimes. The new version introduced is found to be more accurate for all densities and temperatures studied so far, being within a few percent of the MD results in the reduced density rho* less than or similar to 0.7 and comparable to the previous versions for rho* greater than or similar to 0.7 near the triple point.