MODEL FOR DENSITY-FUNCTIONAL THERMODYNAMIC PERTURBATION ANALYSIS OF LENNARD-JONES SOLIDS

Thermodynamic perturbation theory and density-functional approximation s are systematically combined to produce a model of Lennard-Jones soli ds and solid-liquid coexistence. The perturbation theory is based on e xpanding the free energy about that of the fcc hard-sphere solid, whic h is described by an accurate nonperturbative density-functional theor y. Approximations made throughout the development are systematically c hecked against results of Monte Carlo simulations. The Gaussian approx imation used for describing the solid density is shown to be a good ap proximation for the stable solid; however, anisotropies in the structu re that are not captured by the Gaussian approximation become pronounc ed at densities corresponding to metastable solids. The free energies of both the solid and the liquid Lennard-Jones phases and the phase di agram predicted by the density-functional thermodynamic perturbation m odel are in good agreement with Monte Carlo simulations for temperatur es in the range 0.75 less-than-or-equal-to kT/epsilon less-than-or-equ al-to 100.