Calculation of forces in the KKR method

Although the general method of calculating forces on atomic nuclei in the KKR formalism seems to be simple, a closer investigation reveals major challenges: \newline First, Hellmann-Feynman forces are very sensitive to small deviations from a spherical core electron density. Given that spherical symmetry is a requirement for fast convergence of the angular momentum expansion, this contribution needs special treatment. Further, the expression for the interstitial space contribution (i.e., the space outside the Muffin-Tin spheres) is highly sensitive to the angular momentum cut-off, too. \newline We present quantitative studies to the aforementioned problems and trace them back to the underlying mathematical expressions. Based thereupon, we discuss possible improvements to the calculational scheme.