Intermolecular forces are repulsive at short range and attractive at long range (for example, see the Lennard-Jones potential). Since distances between gas molecules are large compared to molecular diameters, the energy of a gas is usually influenced mainly by the attractive part of the potential. As a result, expanding a gas usually increases the potential energy associated with intermolecular forces. Some textbooks say that for gases this must always be the case and that a Joule expansion must always produce cooling. When molecules are close together, however, repulsive interactions are much more important and it is thus possible to get an increase in temperature during a Joule expansion.