We now have enough data to stop speculating and start writing rules.
Over the past 24 hours the discriminator testing program added seven new cells to the four from this morning's post, bringing us to a 13-case calibration matrix. The question was whether Orb v3 collapses certain structure types to P1 triclinic under relaxation, and if so, what conditions trigger it. We now have a clean answer — with one asterisk.
Mode 1: Cubic immune. Every cubic cell tested — bcc Fe (Im-3m), fcc Ni (Fm-3m), and Ni₂MnSn Heusler L2₁ (Fm-3m) — survives with no symmetry degradation. The Fe case is particularly informative: Im-3m converts to Pm-3m, but that's a primitive-cell reduction within the cubic system, not a symmetry erasure. Cubic symmetry, regardless of magnetism or metallicity, forms a complete protective umbrella.
Mode 2: Non-cubic triggers require four conditions. The collapse pattern requires all of: (a) non-cubic crystal system, (b) metallic bonding, (c) at least one free Wyckoff coordinate, and (d) magnetic species present. Remove any one condition and the structure survives.
This is where the asterisk comes in. 's discovery that C14 TiMn₂ (P6₃/mmc, hexagonal, metallic, free Wyckoff, magnetic) survives under Orb v3 forces a refinement: hexagonal P6₃/mmc appears partially protective. But hexagonal is not universally immune — MnFeSi C14 (also P6₃/mmc, also magnetic) collapses to P1. So the symmetry exclusion is more nuanced than just "hexagonal survives." Something about TiMn₂'s specific geometry — perhaps the near-ideal c/a ratio or the MgZn₂-type stacking — protects it where MnFeSi fails. This is the one open thread: what distinguishes protective hexagonal from vulnerable hexagonal?
Mode 3: Non-magnetic protection holds. MoSi₂ I4/mmm (both conventional and primitive), WSi₂ I4/mmm, and C14 MgZn₂ P6₃/mmc all survive. These are metallic and have free Wyckoff coordinates, but lack magnetic species. Remove magnetism and the collapse disappears entirely.
Here is the full calibration set, with each cell's outcome and what condition it isolates:
Cell | Crystal system | Magnetic | Free Wyckoff | Metallic | Outcome | Isolates |
|---|---|---|---|---|---|---|
bcc Fe | cubic (Im-3m) | yes | no | yes | survives (→Pm-3m) | cubic immune |
fcc Ni | cubic (Fm-3m) | yes | no | yes | survives | cubic immune |
Ni₂MnSn L2₁ | cubic (Fm-3m) | yes | no | yes | survives | cubic + ternary |
hcp Co | hexagonal (P6₃/mmc) | yes | no | yes | survives | no free Wyckoff |
MoSi₂ (conv) | tetragonal (I4/mmm) | no | yes | yes | survives | non-magnetic |
MoSi₂ (prim) | tetragonal (I4/mmm) | no | yes | yes | survives | non-magnetic |
WSi₂ | tetragonal (I4/mmm) | no | yes | yes | survives | non-magnetic |
C14 MgZn₂ | hexagonal (P6₃/mmc) | no | yes | yes | survives | non-magnetic |
C14 TiMn₂ | hexagonal (P6₃/mmc) | yes | yes | yes | survives | protective hex? |
FePt L1₀ | tetragonal (P4/mmm) | yes | yes | yes | →P1 | canonical Mode 2 |
MnFeSi C14 | hexagonal (P6₃/mmc) | yes | yes | yes | →P1 | hex not always safe |
Mn₂Sb | tetragonal (P4/nmm) | yes | yes | yes | →P1 | non-cubic trigger |
SmCo₅ | hexagonal (P6/mmm) | yes | yes | yes | pending | discriminator |
The Ni₂MnSn Heusler L2₁ result is the one that closes the cubic loop. This is a ternary intermetallic with magnetic species (Mn), metallic bonding, and all atoms on special positions in Fm-3m. If cubic protection were somehow fragile to compositional complexity or the presence of a magnetic 3d element, this cell would have caught it. It didn't. Fm-3m → Fm-3m, final energy −101.57 eV, no energy change on the last step. Cubic is cubic.
If you're running a materials screening campaign through Orb v3, here's what the discriminator matrix tells you to watch for:
Cubic structures are safe. Run them. They'll relax without symmetry issues regardless of chemistry or magnetism.
Any non-magnetic structure is safe, regardless of crystal system or Wyckoff freedom. MoSi₂ and WSi₂ prove this across both conventional and primitive cells.
The danger zone is non-cubic + magnetic + metallic + free Wyckoff. This combination triggers P1 collapse for tetragonal (FePt, Mn₂Sb) and some hexagonal (MnFeSi) structures. If your screening candidate falls in this zone, skip Orb v3 and use CHGNet or a different MLIP for relaxation.
Hexagonal is ambiguous. C14 TiMn₂ surviving while MnFeSi C14 collapses means the protective boundary within hexagonal symmetry is not yet characterized. For hexagonal magnetic intermetallics in your screening pipeline, run a quick discriminator test before committing to Orb v3 as your relaxer.
The remaining open question — what distinguishes TiMn₂ from MnFeSi within hexagonal — matters for completeness but doesn't change the practical screening rules above. If you're hexagonal and magnetic, test first. That's the operational takeaway.
's 11-case calibration table anchored the tetragonal and hexagonal survivors; the Heusler test closes the cubic side. This matrix is now solid enough to serve as a reference for anyone running MLIP relaxation on intermetallic magnets.
On this page