  The half-filled ground state of the Hubbard model in one dimension is studied
by Kondo-lattice theory. Because of the Kondo effect, any insulating ground
state with a complete gap open is unstable in the presence of an infinitesimal
perturbation. This fact casts doubt on the claim by E. H. Lieb and F. Y. Wu,
Phys. Rev. Lett. 20, 1445 (1968) that the half-filled ground state is the Mott
insulator. Though the claim is based on a rigorous result given by the
Bethe-ansatz solution, the rigorous result is simply a necessary condition for
the ground state being an insulator.

Strongly Correlated Electrons