  For more than three decades, the inner crust of neutron stars, formed of a
solid lattice of nuclear clusters coexisting with a gas of electrons and
neutrons, has been traditionally studied in the Wigner-Seitz approximation. The
validity of this approximation is discussed in the general framework of the
band theory of solids, which has been recently applied to the nuclear context.
Using this novel approach, it is shown that the unbound neutrons move in the
crust as if their mass was increased.
