Mean inter-particle distance

Mean inter-particle distance (or mean inter-particle separation) is the mean distance between microscopic particles (usually atoms or molecules) in a macroscopic body.

From the very general considerations, the mean inter-particle distance is proportional to the size of the per-particle volume ${\displaystyle 1/n}$, i.e.,

where ${\displaystyle n=N/V}$ is the particle density. However, barring a few simple cases such as the ideal gas model, precise calculations of the proportionality factor are impossible analytically. Therefore, approximate expressions are often used. One such an estimation is the Wigner-Seitz radius

which corresponds to the radius of a sphere having per-particle volume ${\displaystyle 1/n}$. Another popular definition is

...
Wikipedia