Electron Correlations in Narrow Energy Bands: Ground State Energy and   Metal-Insulator Transition
  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. 
Leonid Didukh and Yuriy Skorenkyy (Ternopil State Technical   University, Ukraine)