  A method for calculating pair correlation functions in a crystal is
developed. The method is based on separating the one- and two- particle
correlation functions into the symmetry conserving and the symmetry broken
parts. The conserving parts are calculated using the integral equation theory
of homogeneous fluids. The symmetry broken part of the direct pair correlation
function is calculated from a series written in powers of order parameters and
that of the total pair correlation function from the Ornstein- Zernike
equation. The results found for a two-dimensional hexagonal lattice show that
the method provides accurate and detailed informations about the pair
correlation functions in a crystal.

Soft Condensed Matter;Statistical Mechanics