The crust of neutron stars

The structure of the crust of a neutron star is completely determined by the experimentally measured nuclear masses up to a density of the order of 10^11 g.cm^{-3}. At higher densities, the composition of the crust still remains uncertain, mainly due to the presence of ``free'' superfluid neutrons which affect the properties of the nuclear ``clusters''. After briefly reviewing calculations of the equilibrium structure of the crust, we point out that the current approach based on the Wigner-Seitz approximation does not properly describe the unbound neutrons. We have recently abandoned this approximation by applying the band theory of solids. We have shown that the dynamical properties of the free neutrons are strongly affected by the clusters by performing 3D calculations with Bloch boundary conditions.