  A melting transition for a system of hard spheres interacting by a repulsive
Yukawa potential of DLVO form is studied. To find the location of the phase
boundary, we propose a simple theory to calculate the free energies for the
coexisting liquid and solid. The free energy for the liquid phase is
approximated by a virial expansion. The free energy of the crystalline phase is
calculated in the spirit of the Lenard-Jonnes and Devonshire (LJD) theory. The
phase boundary is found by equating the pressures and chemical potentials of
the coexisting phases. When the approximation leading to the equation of state
for the liquid breakes down, the first order transition line is also obtained
by applying the Lindemann criterion to the solid phase. Our results are then
compared with the Monte Carlo simulations.
