{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"9452dedb12da221d473c68f8e3f079385a41c352697ad0b351945b360335e3a5","created":"2026-10-03T07:18:10Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011","c2f73e932b001e465fd6cb1e0857d1ab82e7b70ced32116141d9edf9660d5b92"],"salt":"54d0f7c3088d81571a6b06da521c5772db2de7715d899a94df7271a97a301da7","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"qi.quantum-chemistry-algorithms.femoco-classical-crossover","kind":"well-posed","literature_status":"open","n":"1","parents":[{"note":"","parent_revision":"c2f73e932b001e465fd6cb1e0857d1ab82e7b70ced32116141d9edf9660d5b92","relation":"special_case"}],"plain":"The iron-molybdenum cluster in nitrogenase, the enzyme that turns nitrogen from air into ammonia, was long the flagship target for quantum chemistry on quantum computers. A 2026 classical calculation reached the target accuracy for the standard model, so it is open which larger or harder version still needs a quantum computer.","posed_since":"2017","precise":"Zhai et al. (arXiv:2601.04621, 2026) report the ground energy of the standard 76-orbital, 113-electron FeMoco model to chemical accuracy ($1.6\\,\\mathrm{mHa}$), with several spin isomers degenerate within that accuracy, using unrestricted coupled cluster and DMRG at roughly $10^{5}$-$10^{6}$ CPU core hours, while fault-tolerant estimates for the same model need of order 2000 logical qubits and $10^{9}$-$10^{10}$ Toffoli gates (approximate). Identify a chemically meaningful extension (larger active space, catalytic intermediates, relative energies of the near-degenerate spin isomers) where the classical cost to $1.6\\,\\mathrm{mHa}$ exceeds the quantum cost, with both costs computed explicitly.","problem_ref":null,"references":"","settled_by":"A side-by-side classical calculation and fault-tolerant resource estimate for a specified Hamiltonian showing the crossover.","status_note":"Classical chemical accuracy for the standard model was reported in January 2026; 2025 algorithm improvements reduced quantum Toffoli counts further.","title":"Where classical methods stop for FeMoco-class iron-sulfur 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