  Electronic susceptibilities are a very popular tool to study electronic and
magnetic properties of materials, both in experiment and theory. Unfortunately,
the numerical evaluation of even the bare susceptibility, which depends on the
computation of matrix elements and sums over energy bands, is very
work-intensive and therefore various approximations have been introduced to
speed up the calculations. We present a reliable and efficient implementation
to compute static as well as dynamic bare susceptibilities based on
full-potential density functional theory (DFT) calculations. Based on the exact
results we will assess the accuracy of replacing the matrix elements with a
constant and the impact of truncating the sum over the energy bands. Results
will be given for representative and topical materials, such as Cr, a classical
transition metal, as well as for FeSe and LaFeAsO, examples of iron-based
superconductors.
