  The ground state of the Hubbard model is studied within the single-site
approximation (SSA) and beyond the SSA. Within the SSA, the ground state is a
typical Mott insulator at the critical point n=1 and U/W=+infty, with n being
the electron density per unit cell, W the bandwidth of electrons, and U the
on-site repulsion, and is a normal Fermi liquid except for the critical point.
Beyond the SSA, the normal Fermi liquid is unstable against a non-normal Fermi
liquid state except for a trivial case of U=0 such as a magnetic or
superconducting state in two and higher dimensions. In order to explain actual
observed metal-insulator transitions, one or several effects among the
electron-phonon interaction, multi-band or multi-orbital effects, and effects
of disorder should be considered beyond the Hubbard model.
