Optimization of spherical muffin-tin parameters for real-space multiple-scattering calculations of x-ray spectra

Real-space multiple-scattering theory (RSMS) has been used extensively to calculate x-ray absorption spectroscopy (XAS) in a wide range of solids, liquids and molecules. One of the major benefits of the real-space approach is the capability to describe aperiodic systems such as amorphous materials, liquids, and molecules in addition to periodic structures. One of the reasons for the efficiency and broad applicability of RSMS is the muffin-tin (MT) approximation, where the potential is taken to be spherical near atomic centers and flat in the interstitial region. Several prescriptions have been formulated to define MT potentials [1,2]. While these prescriptions often describe core-level spectra with surprising accuracy, failures are also prevalent, especially in low symmetry systems. Analyses of these failures have been carried out in comparison with non-spherical, full-potential approaches, with the goal of showing improvement over the spherical MT approximation, rather than improvement of the MT approximation. In some cases, the MT radii have been treated as free parameters to be optimized during fits to experimental data. Here we show that an optimization of the MT potentials based on a comparison with accurate density functional theory results can drastically improve the densities of states and XAS in systems with low to intermediate symmetry while preserving the efficiency of the MT approach for XAS. The approach is validated by comparison to experiment and other theoretical methods.