{"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 clusters","topic_ref":"7a5b2e4d74deb25e82b2175151e253081cb18bcfd07a0463c8d69af524e27dcb"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"7ebc3520d6a4c1d83702a205e3560649e19ecf334b2274c7532d587496fef03d","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"35911cdef5c8c2de1c9bf7e2d2971983789852a30b703438e175afe318a36bb8","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"yPu0QoroTyfD1R5Q0QTtVFpCMO61tmVvrwe0iq8ntMVNvyVTDZilqIC9-cOnni8pjU2B7YhT6YUo65eLRW2fBg"},"schema":"pubphys.envelope/1"},"record_hash":"7ebc3520d6a4c1d83702a205e3560649e19ecf334b2274c7532d587496fef03d","leaf_index":2027}