 The Lennard-Jones potential is often used as an approximate model for the isotropic part of a total (repulsion plus attraction) van der Waals force as a function of distance.Van der Waals forces are responsible for certain cases of pressure broadening (van der Waals broadening) of spectral lines and the formation of van der Waals molecules.The London-van der Waals forces are related to the Casimir effect for dielectric media, the former being the microscopic description of the latter bulk property.