Statistical theory for the entropy of a liquid

A correlation expansion for the entropy of a classical monatomic liquid has been found to give an accurate account of the entropy for liquid sodium and for liquid argon. Here, this correlation expansion is given a conceptually simple derivation in the grand-canonical ensemble. The one- and two-particle terms together are shown to accurately represent the entropy of the hard-sphere liquid. The simplicity of the theory suggests a model for an ''ideal liquid,'' whose free energy takes on an especially simple form. The correlation expansion for the entropy is compared in detail with previous related work