  The electron correlations in narrow energy bands are examined in framework of
the Hubbard model. The single-particle Green function and energy spectrum are
obtained in paramagnetic state at half-filling by means of new two-pole
approximation. In the ground state analitical expressions for the energy gap,
polar states concentration and energy of the system are found. Metal-insulator
transitions in the model at change of bandwidth or temperature are
investigated. The obtained results are used for interpretation of some
experimental data in narrow-band materials.
