and Otani reported a small energy barrier, less than 10 meV. [@Hayami10] The values for $\Delta E_{f}$ in Table \[tbl:HfofB50\] were obtained by performing steepest-descent minimization with typically seven or eight iterations, which usually is enough to locate local minimum configurations. However, started from an $8i$ site, we found that continuing the iteration by more than 30 times finally brought the $8i$-site atom to the $4c$ site (see Supplemental Material). In this sense, $8i$ or $8h$ are metastable sites. However, to facilitate comparison with other calculations and experiments, we will still consider these sites as distinct, below.

The configuration B$_{52}$ with two $4c$ sites in the out of plane configuration (two $4c$-site atoms located in different $4c$ planes and not being neighbors) is the lowest-energy configuration. Hereafter, we designate this configuration as B$_{50}$ + 2B$_{4c}$. As mentioned above, all formation energies are defined with respect to this configuration.

Let us now consider only the blocks with $N_{\rm at}=51$ or less in Table \[tbl:HfofB50\]. We see that $\Delta E_{f}$ of many IS are lower than that of B$_{50}$ (3.14 eV). This fact already indicates that these interstitial B atoms are not defects but are a part of the host structure. The 