Initial test runs used Quantum Espresso v.6.4.1 while final relaxation runs used v.7.0. For final relaxations, the kinetic energy cutoff for wavefunctions was 952 eV (70 Ry), and the cutoff for charge density and potential was 6260 eV (460 Ry), both just above the highest recommended values among the pslibrary pseudopotentials. Structure relaxations used a Monkhorst-Pack k-point grid initialized with a minimum of 500/n points (n = number of atoms in the cell) distributed in proportion to reciprocal lattice vector lengths. We required 5×10 -5 eV/atom (3.7×10 -6 Ry/atom) convergence for electronic relaxation, 5×10 -4 eV/atom (3.7×10 -5 Ry/atom) for ionic relaxation, 9.45×10 -3 eV/ Å/atom (3.7×10 -4 Ry/a 0 /atom) for forces on each atom, and 0.5 kbar pressure for cell parameters. The force convergence follows Quantum Espresso's default of 10× the ionic convergence. Similar convergence criteria have performed well for diverse chemical systems. Calculations included spin-orbit coupling but not the magnetic moment of Eu 3+ .