2 photoemission[4], where all the bands are quite renor- malized with respect to density-functional calculations, and the ones lying closest to the Fermi level have a significantly stronger renormalization. An indication of an orbital-selective physics is also provided by the mag- netic response, which seems to be constituted by a metal- lic component (Pauli behavior) and a component asso- ciated to local moments (Curie susceptibility), as if two different kind of carriers are present[5]. OSMT is realized for a large range of the parameter space U − Δ (interaction strength - crystal field split- ting) if the Hund’s coupling J is higher than a critical value. Fig. 1 Sketch of the band structure and splitting that allows the new OSMT mechanism to apply. The three band-model re- alization was studied in Ref. [6]. In this paper we generalize it to the case relevant for the pnictides: 6 electrons in 5 bands. For simplicity we study the case in which one band is lifted and the remaining 4 stay degenerate. 2 The mechanism: selective localization triggered by band degeneracy lifting In a recent paper[6] we have outlined a mechanism for the Orbital-selective Mott transition, which is possibly the most likely to be found in nature, and in particular in the pnictides. This reposes on the different kinetic energy of elec- trons lying in manifolds of bands of unequal degeneracy. In general, even if the bandwidths are equal, the higher the degeneracy, the higher the kinetic energy content. Thus, in case of reasonably decoupled manifolds, inter- mediate interactions can localize only the less mobile electrons, while the more mobile ones remain itinerant. The necessary conditions for this to be realized are thus a suitable splitting of the conduction bands due to the crystal field, and a sizeable Hund’s exchange coupling which suppresses the orbital fluctuations and hence decouples the manifolds. Indeed it is well known that manifolds of degen- erate atomic orbitals are often split in energy by the anisotropy of the Crystal field. Distortions of the ionic structure lower the point group symmetry of the crystal and this changes the Madelung energy of the orbitals oriented along different directions. A typical example is the splitting, induced by tetragonal distortions, of the five d-obitals that generate the conduction bands in transition-metal oxydes in two groups, t$_{2}$$_{g}$ and e$_{g}$ , respectively of three and two bands. We also know from the study of the multi-orbital Hubbard model that Mott insulating phases can arise at large interaction strength for any integer filling[7,8] and it is now clear[9,10,11,12] that the critical interaction strength U$_{c}$ for the Mott transition is in general larger for manifolds of a larger number of bands (e.g. in the SU(N)-orbital Hubbard model it scales with N at large N, while the critical value for the closing of the gap scales with √$_{N}$$_{.) due to the increased kinetic energy of}$ electrons lying in degenerate bands. We analyzed a model in which 4 electrons per site populate three degenerate bands which are split by the crystal field as depicted in Fig. 1 and found that the This situation is ideal for our mechanism to apply, since the upper band is likely to be driven towards half- filling by the crystal field and the U/W value is just about what is needed for a single band to be local- ized by correlations. The top band is relatively close in The crystal field in these compounds is estimated to split the orbitals by putting the xy 130 meV above the degenerate xy and yz , that are in turn 160meV above the 3 z $^{2}$− r 2 and 220meV above the x $^{2}$− y $^{2}$. The Hund’s coupling is estimated of the order of 0 . 35 ∼ 0 . 7 eV while the bandwidth is W ∼ 3 eV and U ∼ 4 eV . Indeed the case relevant for Pnictides[13,14], 6 elec- trons in 5 bands, is even more favorable than the pre- vious one of 4 electrons in 3 bands, since it is closer to global half-filling. Thus an even smaller crystal field splitting can drive the top band to half-filling and to localization. Here we generalize the study performed in Ref. [6] by investigating the realization of the OSMT by the same mechanism, within a more appropriate (albeit still rather caricatured) model for Iron Pnictides. 3 Towards a realistic model for iron pnictides: 6 electrons in 5 bands In practice when J is large enough to suppress the orbital fluctuations, a gap due to U can open in the lifted band, such that the chemical potential falls within it and this band is half-filled. This, and the related lo- calization can happen for the whole range of the crystal field that shifts the gapped band until the chemical po- tential hits one of the borders of the gap.