Spectra for these unit cells are obtained either through querying the MP API or by computing XANES spectra using FEFF 9 with the same parameters as the MP to ensure transferability (catalogued in pymatgen at time of writing as MPXANESSet ). We ultimately computed over 23,000 new XANES spectra to supplement the spectra downloaded from the Materials Project database. Duplicate spectra and those that appear to be from an unconverged calculation (i.e., the computed spectra appear unphysical, such as those with anomalously high peaks in the pre-edge region) are left out from our dataset. The unphysical spectra filtering and de-duplication are similar to the data preparation in earlier work , except here the similarity cutoffs for duplicate data are more restrictive. See SI for the exact removal criteria. In addition, due our focus on interpreting spectral features, we only keep spectra with maximum summed errors less than 0.1 (in absorption units) between individual piecewise polynomial fits and their respective domains in the max-normalized spectrum (See SI).