Density-functional theory of a lattice-gas model with vapour, liquid, and solid phases

We use the classical version of the density-functional theory in the weighted-density approximation to build up the entire phase diagram and the interface structure of a two-dimensional lattice-gas model which is known, from previous studies, to possess three stable phases—solid, liquid, and vapour. Following the common practice, the attractive part of the potential is treated in a mean-field-like fashion, although with different prescriptions for the solid and the fluid phases. It turns out that the present theory, compared to similar theories in the continuum, is of worse quality. Nevertheless, at least a number of qualitative facts are reproduced correctly: (i) the existence of three phases; (ii) the disappearance of the liquid phase when the range of the attraction is progressively reduced; and (iii) the intrusion, just below the triple-point temperature, of a liquid-like layer at the interface between the coexisting solid and vapour phases.