Perturbation weighted-density approximation: The phase diagram of a Lennard-Jones system

We present a density-functional approach for the Helmholtz free energy of simple liquids, based on the separation of attractive and repulsive interactions and within the framework of the weighted-density approximations. The approach reduces to the standard perturbation theory of simple fluids for the homogeneous vapor and liquid phases. It gives a good description of the correlation structure in the crystal, based on a local-compressibility equation. The phase diagram of a Lennard-Jones system is obtained and compared with a computer simulation.