  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.
