The search for a satisfactory theory of simple liquids of spherical
, nonpolar molecules, such as hard spheres or Lennard-Jones molecules, occupied much effort in the first part of the 20th century.
In particular, the period 1953 to 1970 saw the introduction of
integral equation theories and perturbation theories that were
finally successful for such fluids, first for hard spheres and subsequently for liquids of simple molecules with attractive