Possible use of DFT approaches for the determination of double exchange interactions

DFT calculations are performed on a model mixed-valence system presenting a double exchange phenomenon. Due to the intrinsic multireference character of the lowest Ms components of the spin states, it is shown that the interactions involved in the double-exchange model cannot be simply extracted from the DFT energies as it is sometimes done. It is, however, possible to extract from different DFT single determinant energies the interactions of a generalized Hubbard Hamiltonian, from which, in a second step, the double-exchange spectrum may be evaluated. The problems generated by the charge and spin polarization are discussed in both symmetric and non symmetric geometries, and the sensitivity of the results to the choice of the density functional is illustrated.