Fluid mixtures of parallel hard cubes.

The direct correlation function of a fluid mixture of parallel hard cubes is obtained by using Rosenfeld's fundamental measure approximation. This approximation is thermodynamically consistent (compressibility and virial equations of state are equal) and predicts a spinodal instability of the binary mixture for large-to-small side ratio larger than roughly 10, in qualitative agreement with simulations on the lattice version of the model. In two dimensions the system never demix, also in agreement with the simulations.