  We present an ab initio method for eletcronic structure calculations, which
accounts for the interaction of electrons and magnons in ferromagnets. While it
is based on a many body perturbation theory we approximate numerically complex
quantities with quantities from time dependent density functional theory. This
results in a simple and affordable algorithm which allows us to consider more
complex materials than those usually studied in this context ($3d$
ferromagnets) while still being able to account for the non-locality of the
self energy. Furthermore, our approach allows for a relatively simple way to
incorporate self-consistency. Our results are in a good agreement with
experimental and theoretical findings for iron and nickel. Especially the
experimental exchange splitting of nickel is predicted accurately within our
theory. Additionally, we study the halfmetallic ferromagnet NiMnSb concerning
its non-qusiparticle states appearing in the bandgap due to spin-flip
excitations.
