A modified form of the polar model of crystals
  A model for description of materials with narrow energy bands is proposed. It is shown that in narrow-band materials electron-hole symmetry is absent in contrast to the Hubbard model. In this paper metal-insulator transition is studied. The obtained results are compared with experimental data for narrow-band materials. Some specific narrow-band effects are discussed. 
Leonid Didukh (Ternopil State Technical University, Ukraine)