Characteristic quantities of plasmas with interactions
Ionized gases and plasmas

When two particles initially separated by a distance d approach each other, they begin to interact, and if, after this interaction, some measurable change has occurred, we say that a collision has taken place.

Given their electron cloud, neutral particles don't have "billiard ball" collisions, but they do "feel" each other long before they come into physical contact. This interaction is due to the deformation of their electron cloud, resulting firstly in an attractive force (in the case of a Lennard-Jones potential, this force is ~d ^–7 ), then strongly repulsive due to positive nuclei...