Theoretical approach to the correlations of a classical crystal

We present a theoretical approach to the angular-average of the two-body correlation function (g) over tilde(r) for simple solids. It is based on three sum rules for (g) over tilde(r): the virial, compressibility, and normalization equations. We apply the theory to determine this correlation function for the case of the fee solid phase of hard spheres. The agreement with simulation data is excellent over all the density range. The application to other simple systems is discussed. The approach opens a route to perturbation theories for simple solids.