Spin audit of SQD/QSCI quantum-chemistry benchmarks on iron–sulfur clusters

Author here.Background, for anyone who hasn't followed this fight: IBM's iron–sulfur SQD results (Sci. Adv. 2025) are one of the flagship "quantum computers are useful for chemistry now" claims, and a published critique (arXiv:2501.07231) argues the quantum samples never beat classical selected-CI at matched cost. That argument is still live.Both sides have been arguing about energies. Neither measured which electronic state these calculations actually converge to.So I measured it. ⟨S²⟩ comes o…