03-01E

For the restricted primitive model (RPM) we obtain the functional of the grand partition function (within the framework of the collective variables method with a reference system). First we calculate the phase diagram of the RPM in the mean-field (MF) approximation and then we do this calculation taking into account the terms of the higher orders in the effective Hamiltonian. In the both cases the phase diagrams demonstrate the gas-liquid (GL) and charge ordering phase instabilities. In the latter case, the obtained value for the GL critical temperature is in good agreement with the MC simulation data whereas the critical density is underestimated. The explicit expression found for the grand thermodynamic potential in the vicinity of the GL critical point implies a classical critical behaviour of the RPM.