# Methods ## 1. Generation Structures come from **CrystaLLM** (a language model trained on CIF files), prompted with a composition. Most prompts are picked automatically ("autopilot"); a few were typed by a person. `origin` and `prompt` record which. How the autopilot picks prompts changed on 2026-11-01 (`prompt_mode` records it from then on): - before: 30% no composition (CrystaLLM chooses), 70% random cations and counts plus a random anion, with no charge balance. On our own hull data these random formulas were the worst source by far. - since: 45% **substitution** (one element of a stable Alexandria compound swapped for a likely replacement: pymatgen's data-mined substitution probabilities for ions, same-group neighbours otherwise, metal-for-metal in intermetallics; formulas already in the 2.8M-structure training index removed; f-block lanthanides left out because plain PBE handles them poorly), 40% CrystaLLM chooses, 15% random but charge-balanced. ## 2. ML screen Each structure is relaxed with **MACE** (float64). The verdict comes from the formation energy relative to the elements and from how much the cell changed in relaxation: `rock solid` / `plausible` / `borderline` / `unstable` / `falls apart`. ## 3. Novelty Composition and structure are matched (pymatgen StructureMatcher) against a local index of known structures and, since 2026-10-28, live against **Materials Project** and **Alexandria** via OPTIMADE. `known` and `rediscovered` records include the matching ID. Before 2026-10-28 only the local index was used, which missed some known compounds; affected records were re-audited. Since 2026-11-03 a discovery must also be a **composition nobody has reported, in any structure**: - before generation, autopilot formulas (substitution and random modes) are skipped if they appear in a local list of known compositions (the 2024 index above + current Alexandria PBE/PBEsol + both GNoME releases + Materials Project, rebuilt weekly); "surprise" crystals, where CrystaLLM picks the formula, are checked right after generation and skipped before MACE. The README's prompt-mode table counts these skips. - before cataloguing, viable crystals get the online prior-art check described in §7 (about 32 sources incl. OQMD, COD, MPDS, NOMAD, OpenAlex and arXiv). A hit gives novelty `known composition` and no catalogue entry. - a new arrangement of a known formula (the old `new structure` status) is no longer catalogued; the 638 existing ones and every other catalogued record that failed the composition check were re-labelled `known composition` on 2026-11-03, with their old catalogue number and name kept in the novelty record. - `control` prompts (5% of autopilot) ask for known stable compounds on purpose, to check that the generator and MACE still recognise real crystals. They are never catalogued. ## 4. ML convex hull Nightly, catalogued structures get an energy above the hull: MACE energies of the candidate vs MACE-relaxed Alexandria phases near the hull in the same chemical system. `decomposes_to` lists the hull products and fractions. Values below about -0.15 eV/atom are flagged `suspect` (almost always an ML artefact). ## 5. DFT re-check Candidates with ML e_hull ≤ 0.025 eV/atom and ≤ 30 atoms go to DFT, together with their decomposition products on **Alexandria's PBE hull** (≤ 64 atoms each). - Code: Quantum ESPRESSO 6.7, PBE, SSSP 1.1.2 efficiency pseudopotentials; cutoffs = max over the elements of the SSSP-recommended wavefunction/charge cutoffs. - Smearing: Marzari-Vanderbilt, 0.01 Ry. Spin: non-magnetic, except systems containing Fe/Co/Ni/Mn/Cr, which are run spin-polarised from a 2 μB start on those atoms. - Each structure: `vc-relax` on a coarse k-grid (spacing 0.4 Å⁻¹; forces 1e-3 Ry/bohr, pressure 0.5 kbar), then an `scf` on a finer grid (0.25 Å⁻¹, conv_thr 1e-7). Energies come from the final scf only. - Candidate and competitors are computed with identical settings, so the hull is internally consistent. - **Pass**: e_hull ≤ 0.025 eV/atom. That margin is about the size of PBE's typical error and of entropy at room temperature; a pass means "worth a closer look", nothing more. - Some jobs carry a `note` (e.g. a relaxation stopped by hand when it was clearly not going to pass, or an extra competitor checked afterwards). Read it before using the number. ## 6. Properties (passes only) PBE band gap and density of states at the Fermi level; elastic tensor from ±1% strains in the six Voigt directions (ions relaxed at fixed cell), with Born stability check. PBE underestimates band gaps, often by a factor of ~2. ## 7. Phonons (passes only) Since 2026-11-03 a pass first gets a composition check (`dft//novelty.json`, summarised in the `prior_reports` column of `dft_results.csv`): any entry with the same reduced formula, in any structure, in Materials Project, Alexandria, OQMD or COD, or the formula (every element ordering) in a paper title/abstract indexed by OpenAlex. Any hit means no phonon run unless reviewed. This is wider than the structure match in §3: #1856 SrCaCdP2 passed §3 but its structure is OQMD entry 5879242 (a computed prediction, no ICSD record). phonopy finite displacements on a supercell with every side ≥ 7 Å, after a tight fixed-cell relaxation; frequencies on a 10×10×10 mesh. Any mode softer than -0.1 THz away from the Γ acoustic modes = unstable. The supercell is small, so long-wavelength instabilities can be missed. ## Known limits - Hulls only include competitors that exist in Alexandria (and, for the ML hull, phases near its hull). A missing competitor can make a candidate look more stable than it is. - PBE alone; no van der Waals, no Hubbard U, no finite-temperature free energies. - Pass ≠ synthesisable. No candidate here has been made in a lab.