  Transition-metal substitution in Fe pnictides leading to superconductivity is
usually interpreted in terms of carrier doping to the system. We report on a
density functional calculation of the local substitute electron density and
demonstrate that substitutions like Co and Ni for Fe do not carrier dope but
rather are isovalent to Fe. We find that the extra d electrons for Co and Ni
are almost totally located within the muffin-tin sphere of the substituted
site. We suggest that Co and Ni act more like random scatterers scrambling
momentum space and washing out parts of the Fermi surface.
