Validity of the Wigner-Seitz approximation in neutron star crust

Since the seminal work of Negele and Vautherin, the Wigner-Seitz approximation has been widely applied to study the inner crust of neutron stars formed of nuclear clusters immersed in a neutron sea. In this article, the validity of this approximation is discussed in the framework of the band theory of solids. For a typical cell of Zr-200, present in the external layers of the inner crust, it is shown that the ground state properties of the neutron gas are rather well reproduced by the Wigner-Seitz approximation, while its dynamical properties depend on the energy scale of the process of interest or on the temperature. It is concluded that the Wigner-Seitz approximation is well suited for describing the inner crust of young neutron stars and the collapsing core of massive stars during supernovae explosions. However, the band theory is required for low temperature transport properties as, for instance, the effective neutron mass giving rise to entrainment effects.