  FeGe in the B20 phase is an experimentally well-studied prototypical chiral
magnet exhibiting helical spirals, skyrmion lattices and individual skyrmions
with a robust length of 70~nm. While the helical spiral ground state can be
verified by first-principles calculations based on density functional theory,
this feature size could not be reproduced even approximately. To develop a
coherent picture of the discrepancy between experiment and theory, we
investigate in this work the magnetic properties of FeGe from first-principles
using different electronic-structure methods. We study atomistic as well as
micromagnetic parameters describing exchange and Dzyaloshinskii-Moriya
interactions, and discuss their subtle dependence on computational, structural,
and correlation parameters. In particular, we quantify how these magnetic
properties are affected by changes of the lattice parameter, different atomic
arrangements, exchange and correlation effects, finite Fermi-function
broadening, and momentum-space sampling. In addition, we use the obtained
atomistic parameters to determine the corresponding Curie temperature, which
agrees well with experiments. Our results indicate that the well-known and
well-accepted relation between the micromagnetic parameters and the period of
the helical structure, is not valid for FeGe. This calls for new experiments
exploring the relation by measuring independently the spin stiffness, the
spiralization and the period of the helical spin spiral.
