  The lattice system with competing interactions that models biological objects
(colloids, ensembles of protein molecules, etc.) is considered. This system is
the lattice fluid on a square lattice with attractive interaction between
nearest neighbours and repulsive interaction between next-next-nearest
neighbours. The geometric order parameter is introduced for describing the
ordered phases in this system. The critical value of the order parameter is
estimated and the phase diagram of the system is constructed. The simple
quasi-chemical approximation (QChA) is proposed for the system under
consideration. The data of Monte Carlo simulation of equilibrium properties of
the model are compared with the results of QChA. It is shown that QChA provides
reasonable semiquantitative results for the systems studied and can be used as
the basis for next order approximations.
