  The statistical mechanical basis of the fluctuation theory of mixtures is
reviewed. An overview of the statistical mechanical relations between the
microscopic properties of a system and its macroscopic properties is presented.
The distribution functions in equilibrium systems are defined and relations
between the distribution functions and thermodynamic properties for pure fluids
and fluid mixtures are reported. The available approximations to distribution
functions are presented. Definitions of direct correlation functions, direct
correlation function integrals, and their relations with fluctuation integrals
are reported.
