  We have calculated the local magnetic susceptibility of one of the
prototypical Fe-based superconductors (LaFeAsO) by means of the local density
approximation + dynamical mean field theory as a function of both (imaginary)
time and real frequencies with and without vertex corrections. Vertex
corrections are essential for obtaining the correct $\omega$-dependence, in
particular a pronounced low-energy peak at $\omega \sim 0.2 $eV, which
constitutes the hallmark of the dynamical screening of a large instantaneous
magnetic moment on the Fe atoms. In experiments, however, except for the case
of x-ray absorption spectroscopy (XAS), the magnetic moment or the
susceptibility represent typically the average over long time scales. In this
respect, the frequency range of typical neutron experiments would be too
limited to directly estimate the magnitude of the short-time moment.
