  Experimental evidence both for and against a half-metallic ground-state of
the Heusler compound Co2FeSi has been published. Density functional theory
based calculations suggest a non half-metallic ground state. It has been
argued, that on-site Coulomb interaction of the d electrons has to be taken
into account via the LDA+U method, which predicts a half-metallic ground-state
for U = 2.5...4.5 eV. X-ray magnetic linear dichroism (XMLD) can be used as a
tool to assess the appropriateness of the LDA+U approach: the calculated
spectra within the LDA+U or GGA+U schemes are different from those within the
LDA or GGA. Due to its ability to separate different orbital symmetries, XMLD
allows us to distinguish between different models of the electronic structure
of Co2FeSi. In this article we discuss experimental XMLD spectra and compare
them with detailed first principles calculations. Our findings give evidence
for the inadequacy of the LDA+U or GGA+U band structures, whereas constrained
calculations with the GGA and a fixed spin moment of 6 \mu_B give better
overall agreement between experiment and theory.
