The self-consistent diagram approximation for lattice systems

We apply the self-consistent diagram approximation to calculate equilibrium properties of lattice systems. The free energy of the system is represente d by a diagram expansion in Mayer-like functions with averaging over states of a reference system. The latter is defined by one-particle mean potentia ls, which are calculated using the variational condition formulated. As an example, numerical computations for a two-dimensional lattice gas on a squa re lattice with attractive interaction between nearest neighbours were carr ied out. The critical temperature, the phase coexistence curve, the chemica l potential and particle and vacancy distribution functions coincide within a few per cent with exact or with Monte Carlo data.