Correlation functions in liquids and crystals: Free-energy functional and liquid-to-crystal transition

A free-energy functional for a crystal that contains both the symmetry-conserved and symmetry-broken parts of the direct pair-correlation function has been used to investigate the crystallization of fluids in three dimensions. The symmetry-broken part of the direct pair-correlation function has been calculated using a series in ascending powers of the order parameters and which contains three- and higher-body direct correlation functions of the isotropic phase. It is shown that a very accurate description of freezing transitions for a wide class of potentials is found by considering the first two terms of this series. The results found for freezing parameters including the structure of the frozen phase for fluids interacting via the inverse power potential u(r) = epsilon (sigma/r)(n) for n ranging from 4 to infinity are in very good agreement with simulation results. It is found that for n > 6.5 the fluid freezes into a face-centered cubic (fcc) structure while for n <= 6 the body-centered cubic (bcc) structure is preferred. The fluid-bcc-fcc triple point is found to be at 1/n = 0.158, which is in good agreement with simulation result.