A density functional approach to freezing transitions in molecular fluids: Dipolar hard spheres

Abstract

We extend a density functional theory of atomic systems to simple molecules using first-order perturbation theory. We model the hydrogen halides as dipolar hard spheres and consider freezing into two orientationally different fcc lattices. In one of these, the dipoles may align parallel to a single, space-fixed axis while in the other, the dipoles in adjacent faces of the fcc unit cell are constrained to lie perpendicular to one another. The second structure approximates the experimentally observed molecular crystals of HF and HCl, and the phase diagram calculated from this model agrees qualitatively with experiment. At high temperatures, or for weak enough dipole moments, a plastic (orientationally disordered) solid precedes the molecular crystal. The plastic phase is unaffected by the dipolar perturbation but eventually becomes a molecular crystal upon lowering the temperature or increasing the density. We also discuss second-order perturbation theory, the effects of higher multipoles, and some general consequences of the density functional theory of molecular systems.