  Here, we investigated the interrelationship between pressure and correlation
in the LaFeAsO compound by the density functional theory method combined with
the dynamical mean field theory (DFT+DMFT) method. The spectral function and
the occupation number (ON) of Fe-3d orbitals at different pressures were
extracted from the calculations, and the importance of the role of correlation
in superconductivity in iron-based compounds was indicated. The measured ON of
Fe-3d orbitals demonstrated charge transfer between them and also the crucial
role of the Fe-3dxy (xy) orbital in superconductivity in these materials. A
switching behavior between high and low TC through decreasing the correlation
was observed in our calculations, originating from the orbital switching. This
shows the significant role of the orbital degrees of freedom in iron-based
superconductors.
