Characteristic values
Thermal plasmas: fundamental aspects

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 really have a "billiard ball" collision, but they do "feel" each other before making 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 proportional to d ^-7 ), then strongly repulsive due to the positive nuclei (for a Lennard-Jones potential, this force is proportional to d ^-13 ...