  The angle-resolved photoemission spectroscopy, ARPES, is able to measure both
the spectra and spectral weights of the quasiparticles in solids. Although it
is common to interpret the band structure obtained within the
density-functional-theory based methods as quasi-particle spectra, these
methods are not able to provide the changes in spectral weights of the electron
excitations. We use Vienna Ab initio Simulation Package, VASP, for evaluation
of the quasiparticle spectra and their spectral weights within Hedin GW
approximation for Fe3Si and FeSi2, providing, thus, a prediction for the ARPES
experiments. Comparison of the band structures shows that both theories reflect
peculiarities of the crystal structures in similar shape of the bands in
certain k-directions, however, in general the difference is quite noticeable.
We find that the spectral weight of quasiparticles in these compounds deviates
from unity everywhere and shows non-monotonic behavior in those parts of bands
where the delocalized states contribute to their formation. Both methods lead
to the same conclusion: those of iron ions in Fe3Si which occupy the positions,
where they are surrounded by only Fe ions, have the d-electrons localized and
large magnetic moment whereas Fe ions with the Si nearest neighbors have
d-electrons delocalized and the magnetic moments quenched. The Si influence on
the Fe ion state is even more pronounced in the FeSi2 where all iron ions have
the Si ions as nearest neighbors: both approaches produce zero moment on iron
ions. The advantages and disadvantages of both approaches are discussed.
