Abstract
<jats:p>Hoppe's additivity theorem predicts that the MAPLE of a ternary oxide should equal the weighted sum of its binary oxide components to within one percent; despite its diagnostic potential, this theorem has never been tested at database scale. We present MaplePy, an open-source tool for automated MAPLE calculation from crystallographic information files, and apply it to 1,410 perovskite and pyrochlore structures from the ICSD. For perovskites, 70.5% satisfy the one-percent threshold; for pyrochlores, 62%. A seven-category failure classification separates genuine violations from metadata artefacts, oxidation state errors, and coordinate errors. Of 1,380 testable structures, one in six (234, 17%) carries a detectable problem. Thirtyone entries from at least eighteen publications spanning 2006–2026 carry coordinate errors so severe that the deposited structures are physically wrong, yet all escaped peer review. Rare earth nickelates show systematic 1.7–2.8% residuals consistent with Ni–O chargetransfer covalency invisible to bond valence sum analysis; the rare earth titanate pyrochlore series maps the onset of the pyrochlore-to-fluorite order–disorder transition through correlated residuals and Madelung factors. MaplePy and the binary oxide reference table are released as open-source tools.</jats:p>