  A previously developed fundamental measure fucntional [J. Chem. Phys.
vol.107, 6379 (1997)] is used to study the phase behavior of a system of
parallel hard cubes. The single-component fluid exhibits a continuous
transition to a solid with an anomalously large density of vacancies. The
binary mixture has a demixing transition for edge-length ratios below 0.1.
Freezing in this mixture reveals that at least the phase rich in large cubes
lies in the region where the uniform fluid is unstable, hence suggesting a
fluid-solid phase separation. A method is develop to study very asymmetric
binary mixtures by taking the limit of zero size ratio (scaling the density and
fugacity of the solvent as appropriate) in the semi-grand ensemble where the
chemical potential of the solvent is fixed. With this procedure the mixture is
exactly mapped onto a one-component fluid of parallel adhesive hard cubes. At
any density and solvent fugacity the large cubes are shown to collapse into a
close-packed solid. Nevertheless the phase diagram contains a large
metastability region with fluid and solid phases. Upon introduction of a slight
polydispersity in the large cubes the system shows the typical phase diagram of
a fluid with an isostructural solid-solid transition (with the exception of a
continuous freezing). Consequences about the phase behavior of binary mixtures
of hard core particles are then drawn.
