THE IDEAL CRYSTAL AS A CRYTERION OF THE ACCURACY FOR THE APPROXIMATE EQUATIONS ON THE LIQUIDS. A.G. Balakchi and Yu.V. Agrafonov, Department of Physics, Irkutsk State University, 20 Gagarin Blvd.,Irkutsk, 664003, Russia.

It is necessary for the statistical discription of collrctive effects in liquids to set that or other approximation between direct and pair correlation functions. Nowdays it is known about 20 approximate integral equations of the Ornsteine-Zernike (OZ) type connecting the direct and pair correlation fanctions. The degree of accuracy of every such approximation is impossible to be evaluated. The physical meaning of their basic approximations has not been cleared up to the end yet.

In our work we suggest using the generalized OZ equation for the discription of the ideal crystal. The pair correlation fanction in this limit case takes the meaning of the Dirak function. As a result, a linear integral equation for the direct correlation fanction is achieved, which has a simple analytical solution. It should fit the resalts known from crystallophysics for the ideal cristal. It is shown that in this limit case the neighbouring order disappear and the direct correlation function discribes a distant order which is typical for the ideal crystal. We suppose that the approximation correctly discribing of the limit tranzisition from the liqoid state to the solid state at T=0 will have the physics meaning. So the limit transition to the model of the ideal crystal may be considered as a physical criterion for the evaluation of the accuracy for the approximations used in physics of liquids.