Orbital ordering in manganites in the band approach
  We consider the orbital ordering in LaMnO3 and similar systems, proceeding from the band picture. We show that for the realistic magnetic structure of A-type there exists a complete nesting betweeen two e_g-bands. As a result there occurs an instability towards an excitonic insulator-like state -- an electron-hole pairing with the wave vector Q=(\pi,\pi), which opens a gap in the spectrum and makes the system insulating. In the resulting state there appeasr an orbital ordering -- orbital density wave (ODW), the type of which coincides with those existing in LaMnO3. 
D.V. Efremov, D.I. Khomskii