Linear, diatomic crystal: single-electron states and large-radius   excitons
  The large-radius exciton spectrum in a linear crystal with two atoms in the unit cell was obtained using the single-electron eigenfunctions and the band structure, which were found by the zero-range potential (ZRP) method. The ground-state exciton binding energies for the crystal in vacuum appeared to be larger than the corresponding energy gaps for any set of the crystal parameters. 
Vadym M. Adamyan, Oleksii A. Smyrnov