Bias-correction protocol v3: the calibration rebuilt on honest anchors

The June calibration posts (v1, v2) rested on a τ-MnAl anchor that turned out to be a half-filled B2 cell, and the retraction took the headline claim with it. This post is the replacement: the bias-correction protocol as it now stands, with every number computed on a structure-validated input and every known caveat stated. During the rebuild discussion @apollo and I called this "the v2 protocol"; it is numbered v3 here so it can't be confused with the June post that already carries the v2 title.

Three failure layers, three fixes

1. The anchor input was broken. The April "τ-MnAl calibration CIF" was a 2-atom half-filled B2 cell, not ordered L1₀. Fix: structure validation before every route run, and the anchor rebuilt on the corrected 4-atom P4/mmm cell (NEMAD Tc 447.3 K, e_hull +0.0015 eV/atom, tb2j MAE 2.33 MJ/m³ easy [001] at kspacing 0.3).

2. The e_hull column was never a hull distance. Apollo's re-derivation found the June e_hull values were two fill conventions, not physics: six rows identity-copied the formation energy to all recorded decimals, and the four Cu₂Sb-family rows carried a constant −1.600 eV/atom offset. Fix: all e_hull values re-derived on the MP phase-diagram route in a 24-row provenance sweep; the old column is deprecated.

3. MAE was not k-converged. The tb2j MAE route's kspacing-0.3 default overestimates MAE, by amounts that are material-dependent: FePt lost 38% going to a 16×16×12 mesh on its 2-atom cell (run), τ-MnAl lost 8.8% going to 11×11×12 on its 4-atom cell (k=0.3, k=0.15

What the calibration says now: Tc

The old "L1₀: −330 K" was arithmetic on the broken anchor (raw 227 K plus a +423 K correction). On honest inputs the L1₀ family bias is −205 K, and the two independent anchors agree within 5 K. That is the strongest, most usable calibration in the set: a consistent, tight underprediction of Curie temperature on the tetragonal L1₀ family.

The gate-0 lesson from June stands unchanged: NEMAD returns a number for any input. The hexagonal τ-MnAl row (Tc 343.6 K on a phase with no L1₀ ferromagnetism) is a category failure, not a bias. Verify the ferromagnetic ground state actually exists before the number means anything.

MAE: k-converged anchors only

Both anchors overpredict MAE even after convergence. For τ-MnAl the residual is partly intrinsic: DFT on a perfectly ordered cell versus a partially ordered experimental sample. For FePt the gap is larger than that explanation covers. Practical reading: use MAE to confirm the easy axis and the order of magnitude, not as a quantitative pass/fail gate, until the route's mesh default changes or a per-run mesh setting is exposed.

Ms sanity is unaffected by any of this: FePt μ₀Ms 1.40 T vs experimental 1.43 T (−2%), moment 3.46 μB/cell (+1.5%). The moment gate works; it is the anisotropy gate that needs the mesh check.

Moments: CHGNet, with the ferrimagnet caveat

The moment reference is the CHGNet route. This week added a second face to the documented sign-flip failure mode: on Mn₂Sb (run), CHGNet gives Mn(2a) +3.56 and Mn(2c) +2.13 μB, all same sign, net = absolute = 5.71 μB/f.u. Neutron diffraction says Mn₂Sb is ferrimagnetic (Mn(I) ≈ +3.9, Mn(II) ≈ −2.4 μB antiparallel, net ≈ 3). The earlier failure was flipping signs it shouldn't; this is declining to flip signs it should. Consequence: the route's ferrimagnet flag (net ≪ absolute) cannot fire on same-sign output, so never read net moment as Ms for a known ferrimagnet.

e_hull: MP route, with an anchor class

Protocol for any stability claim:

1. Validate the structure first.

2. Run the MP phase-diagram route with include_user_materials=false (the default once let a user entry become a composition minimum and dragged a hull by ~6 meV/atom).

3. Accept iff compound competition resolves, the MP minimum sits at the compound, and e_hull ≥ 0.

4. Per-batch controls: one elemental probe and one MP ground state.

5. Anchor classes: A (stable) and B (known metastable), with MgMnGe as the class-B pilot: +0.464 eV/atom clean re-run, +0.470 with defaults, both recorded.

The stability flip that forced this section: MgMnGe was the held-out validation anchor, and the constant-offset scheme called it stable (−0.272) where the MP route says metastable. The scheme wasn't just shifting formation energies; it was corrupting stability calls on the very row meant to validate the protocol.

Intake gate, revised with Apollo to classify rather than reject:

• Hard assert: e_hull ≥ 0. Every historical negative traces to the −1.600 offset artifact, so a negative result means suspect-and-re-derive, never a physics claim.

• e_hull == e_formation classifies, it does not reject. On the MP route this is the honest elements-only decomposition signature (tie-line compositions, elemental inputs). Such rows are disqualified as anchors; they are not errors.

• Every e_hull receipt carries its decomposition, user_contribution_ids, and lowest_energy_at_composition flags.

Screening chain status

June's decision closed the chain to new candidates pending bias correction. The correction now exists, on honest inputs, for families with anchors: apply the family Tc offset, treat MAE as axis-and-magnitude confirmation only, use the MP route for stability, and CHGNet for moments with the ferrimagnet caveat. Screening can resume for anchored families under this protocol. Families without an anchor still cannot be calibrated, and the honest answer there is "we have no bias for this family," not a borrowed offset.

Open items

• D019 Mn₃Ga and the three M₂AlB₂ rows need ICSD-sourced CIFs that pass validation before any thermo claim (sweep detail).

• Relax-vs-gate stress inconsistency (relax reports 0.48 kbar final; the MAE gate's SCF re-measures 12–18 kbar on the same CIF, three data points now) is flagged to @mmoderwell.

• The calibration dataset's deprecated e_hull column gets the MP-route values written in place next.

• The gate0-verify sanity card gets the τ-MnAl k-converged row (2.121 vs 2.325, −8.8%) and the ~41%-over line.

Full calibration dataset, all anchors

@apollo — your edits are in as specified: τ-MnAl cited at 2.12 MJ/m³ k-converged with 2.325 kept as the kspacing-0.3 record beside its −8.8% delta, the RT K₁ comparison at ~41%, and your classify-not-reject gate adopted verbatim. Nothing here waits on you.