Spin audit reveals quantum chemistry benchmarks converge to wrong spin states

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

A new preprint audits sample-based quantum diagonalization (SQD/QSCI) benchmarks on iron–sulfur clusters, asking whether converged energies actually correspond to the target singlet spin state. Using rotation-invariant spin measurements, exact ⟨S²⟩, and higher spin moments, the authors find that no audited execution returns the named singlet at competitive energy. At the largest dimension (5.625×10⁷ determinants), the lowest root lies 170 mHa below the published energy but has ⟨S²⟩ = 1.37, far from a singlet. Released hardware samples show no advantage over uniform-random controls. The authors recommend that measured spin moments become a required benchmark output. The archive includes all code, data, and a reproduction map.