Microscopic description of bond orientational order in simple liquids

A microscopic approach to the transition of isotropic liquid into an anisotropic phase is suggested. Such an anisotropic liquid phase appears in modern phenomenological theories of two-dimensional melting. The approach is based on the investigation of the closed exact nonlinear integral equation for conditional distribution functions. The isotropic phase instability criterion is obtained. The long-range behaviour of the correlation function of the orientational fluctuations of the pair distribution function is analysed. An attempt is made to estimate the instability point in systems of hard disks and hard spheres.