A refined model of the double exchange phenomenon:: Test on the stretched N2+ molecule

The N-2(+) molecule is studied at different interatomic distances as a model molecule for the double exchange mechanism. The energy spectrum as well as the wave functions of the lowest states are analyzed and confronted both with the usual model of double exchange and with a recently proposed refined model. It is shown that the usual model fails to reproduce the energy spacings while the refined model is valid on a large domain of interatomic distances (in the magnetic regime). The study of a model molecule on a large domain of interatomic distances makes it possible to systematically investigate several regimes associated with different energetic state orderings. The perfect agreement between the refined model and the computed energies in the whole domain of stretched distances shows its applicability to a large number of real compounds. Finally, the respective contributions of dynamical and nondynamical correlations are analyzed. (C) 2004 American Institute of Physics.