  Based on first-principles density functional calculations, a general approach
for determining and analyzing the degree of spin polarization (P) in
ferromagnets is presented. The approach employs the so-called tetrahedron
method to evaluate the Fermi surface integrations of P in both ballistic and
diffusive regimes. The validity of the method is examined by comparing the
calculated P values for Fe and Ni with the experiment. The method is shown to
yield highly accurate results with minimal computational effort. Within our
approach, it is also possible to systematically analyze the contributions of
various types of electronic states to the spin induced transport. As a case
study, the transport properties of the soft-ferromagnet CeMnNi4 are
investigated in order to explain the origin of the existing difference between
the experimental and theoretical values of P in this intermetallic compound.
