Accurate bare susceptibilities from full-potential ab initio calculations

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 such calculations. We present a reliable and efficient implementation of the tetrahedron method which allows us to accurately calculate both static and dynamic bare susceptibilities, based on full-potential density functional theory (DFT) calculations. In the light of the exact results we assess the effects of replacing the matrix elements by 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 the iron-based superconductor FeSe.