01-09U

We study thermodynamic and dielectric properties of ferroelectric KH$_2$PO$_4$ type ferroelectrics within the framework of the proton ordering modelo with taking into account the tunneling effects. Within the cluster approximation for the free energy we calculate the components of the static dielectric susceptibility tensor, obtain and study the system of equations for the proton ordering parameter, equation for the Curie temperature, as well as Curie-Weiss temperature and constant. We propose an approach for determination of the optimal values of the model parameters that permit to properly describe experimental data for thermodynamic and dielectric characteristics of the KH$_2$PO$_4$\ type crystals. We find such values of the KH$_2$PO$_4$\ crystal and obtain a very good agreement between the theory and experiment. A character of unphysical behavior of the cluster approximation results is studied.