Lorenz gas
Encyclopedia
A Lorentz gas is a model for gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

es where particle collisions are considered, as opposed to the Knudsen gas
Knudsen gas
A Knudsen gas is a model for gases where particle collisions are ignored, as opposed to the Lorenz gas where collisions are considered. In these gases the mean free path of molecules is bigger than the receptacle where the gas is contained...

which ignores particle collisions. http://wcms1.rz.tu-ilmenau.de/fakmn/fileadmin/template/ifm/user/Babovsky/Preprints/preprint04_24.pdf
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