Density functional theory of nonuniform polyatomic systems. I. General formulation

We extend the density functional theory of nonuniform fluids to the cases of systems composed of polyatomic species. By the method of Legendre transforms, one demonstrates the existence of a free energy density functional where the densities refer to the locations of interaction sites (not full molecular coordinates). A variational principle for the free energy is derived. The methodology retains nearly all the mathematical simplicity of the traditional theory of atomic fluids. Thus, it may provide a practical route to deriving mean field theories of assembly and phase transitions in complex systems. Certain nonlinearities intrinsic to polyatomic systems and absent in simple fluids become apparent in our analysis. These features are associated with the entropy density functional for systems with bonding constraints. They must be carefully assessed in accurate applications.