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In order to not miss crucial seed structures which ultimately led to the experimental structure, we found that the randomly generated initial population of structures must be sufficiently large. An initial population of size 300 was sufficient with a single generation size of 60. In total $\sim 700$ metastable structures were produced in 8 generations. In addition, spin polarization is crucial for the local relaxations performed within each USPEX generation in order to find the experimental structure. When non-spin-polarized DFT was used, we could not find some of the lowest energy materials (including the observed structure)  To understand the behavior of the energies as a function of $U$, we first roughly grouped the structures by the behavior of the $E$ vs. $U-J$ curve, which we have indicated by colors in Fig.~\ref{fig:reordering}. The largest group has a slope of roughly $∆E \sim 0.3U$. A second subset has energies that are relatively constant $(∆E \sim const)$. \text{const})$.  The third group, which appeared to have the lowest energies in the U = 0 run, rapidly increases in energy with $∆E \sim 0.7U$. In order to rationalize this behavior, we begin by examining the density matrix of the 3$d$ orbitals produced by the DFT calculations and magnetization per Co atom. The Coulomb $U$ in the LDA+U formalism penalizes non-integer occupancies by coupling to $tr(n_\sigma - n_\sigma^2)$, where $n_\sigma$ is the density matrix of the 3$d$ orbitals. For the case of diagonal density matrices, this term indeed vanishes when the orbital occupancies are 0 or 1, and is maximal at $1/2$. We have tabulated the diagonal elements of $n_\sigma$ in Table~\ref{tbl:bacoso}, and we find structures with itinerant electronic behavior, i.e. non-integer density matrices, to be more strongly penalized as $U$ increases. The experimental structure has nearly integer occupancies and thus remains low-energy while the energies of all other structures increase.