For structure relaxation, we tested pseudopotentials from the pslibrary set (v. 1.0.0) 10 that allow for spin-orbit coupling contributions and ones from the SSSP Efficiency (v. 1.1.2) set that use the scalar relativistic approximation. The pslibrary europium pseudopotentials placed the 4f electrons in the core while the SSSP europium pseudopotential placed them in the valence. For dielectric function calculations, Quantum Espresso requires norm-conserving pseudopotentials, so we used pseudopotentials from the Pseudo-Dojo set. Band structures calculated with the Pseudo-Dojo pseudopotentials matched those calculated with the pslibrary pseudopotentials. All pseudopotentials are listed with their recommended energy cutoffs in the Supporting Information, as are the valence configurations of each europium pseudopotential.