  We present an ab-initio study of Ru substitution in two different compounds,
BaFe2As2 and LaFeAsO, pure and F-doped. Despite the many similarities among
them, Ru substitution has very different effects on these compounds. By means
of an unfolding technique, which allows us to trace back the electronic states
into the primitive cell of the pure compounds, we are able to disentangle the
effects brought by the local structural deformations and by the impurity
potential to the states at the Fermi level. Our results are compared with
available experiments and show: i) satisfying agreement of the calculated
electronic properties with experiments, confirming the presence of a magnetic
order on a short range scale; ii) Fermi surfaces strongly dependent on the
internal structural parameters, more than on the impurity potential. These
results enter a widely discussed field in the literature and provide a better
understanding of the role of Ru in iron pnictides: although isovalent to Fe,
the Ru-Fe substitution leads to changes in the band structure at the Fermi
level mainly related to local structural modifications.
