  We calculate the strength of the frequency-dependent on-site electronic
interactions in the iron pnictides LaFeAsO, BaFe2As2, BaRu2As2, and LiFeAs and
the chalcogenide FeSe from first principles within the constrained random phase
approximation. We discuss the accuracy of an atomic-like parametrization of the
two-index density-density interaction matrices based on the calculation of an
optimal set of three independent Slater integrals, assuming that the angular
part of the Fe-d localized orbitals can be described within spherical harmonics
as for isolated Fe atoms. We show that its quality depends on the ligand-metal
bonding character rather than on the dimensionality of the lattice: it is
excellent for ionic-like Fe-Se (FeSe) chalcogenides and a more severe
approximation for more covalent Fe-As (LaFeAsO, BaFe2As2) pnictides. We
furthermore analyze the relative importance of different screening channels,
with similar conclusions for the different pnictides but a somewhat different
picture for the benchmark oxide SrVO3: the ligand channel does not appear to be
dominant in the pnictides, while oxygen screening is the most important process
in the oxide. Finally, we analyze the frequency dependence of the interaction.
In contrast to simple oxides, in iron pnictides its functional form cannot be
simply modeled by a single plasmon, and the actual density of modes enters the
construction of an effective Hamiltonian determining the low-energy properties.
