  The chemical ordering transition in a binary alloy is examined using
classical density functional theory for a binary mixture. The ordered lattice
is assumed to be obtained from the disordered lattice by a volume change only,
as in L1_2 ordering from an face centered cubic chemically disordered crystal.
Using the simplest possible approach, second order truncation of the expansion,
non-overlapping Gaussian distributions at the sites, and expansion of the
correlation functions about the sites, a very tractable expansion is obtained.
Under these assumptions the expansion consists of the same terms as the lattice
gas formalism where the lattice is implicitly taken as fixed, plus additional
interaction terms, and an additional entropy term. This additional entropy term
represents a lowest order approximation to the vibrational entropy change.
