  We have analysed the unusual electronic structure of Sr_2FeMoO_6 combining
ab-initio and model Hamiltonian approaches. Our results indicate that there are
strong enhancements of the intraatomic exchange strength at the Mo site as well
as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss
the possibility of a negative effective Coulomb correlation strength (U_{eff})
at the Mo site due to these renormalised interaction strengths.
