# LJ units to SI

Hello everyone,

I am using LAMMPS with LJ units and have calculated diffusion. I am aware that LJ units are unitless, but I am wondering how I can convert my diffusion results to real substances.
I read the LAMMPS and Units documentation but struggled to convert the model to SI units. I couldn’t find a clear example to follow.
For instance, I would like to transfer a model that calculates the diffusion of CO2 in SI units.
the following model is from “Yu Zhu”, “Prediction of diffusion coefficients for gas, liquid, and supercritical fluid: application to pure real fluids and infinite dilute binary solutions based on the simulation of Lennard–Jones fluid”
Eq:
D^*_{LJ}=\frac{3}{8\sqrt{\pi}}\frac{\sqrt{T^*}}{n^*}\times\Big(1-\frac{n^*}{a(T^*)^b}\Big)\Big[1+(n^*)^c\Big(\frac{d(n^*-1)}{e(n^*-1)+(T^*)^{f+gn^*}}+h\Big)\Big]\times\exp\Big(-\frac{n^*}{2T^*}\Big)

He calculated for CO2, but I was wondering how can we do that?

First off, this is not a question about LAMMPS but about some science topic and thus off-topic for this category. That you have used LAMMPS is not relevant. I will thus re-categorize your post to the “Science Talk” category.

Second, there have been multiple discussions of the topic of converting reduced units to “real” units in the past with explanations and suggestions for further reference. You can search the forum archives for them.