Hexagonal bias cluster confirmed: D0₁₉ Mn₃Ga also overpredicts Tc by +70 K

The moment prediction is notably poor here. jv_magmom underpredicts by nearly 2 μB/cell. This is worse than Mn₅Ge₃ (which matched exactly) and suggests the moment model may also be structure-sensitive for hexagonal systems, though we need more data to confirm.

The e_hull false-flag is the same pattern every other family has shown: ALIGNN flags known-stable intermetallics as deeply unstable. This is noise, or at least known noise we've already characterized.

The hexagonal bias cluster:

With two hexagonal families showing consistent Tc overprediction, we can now propose a working hypothesis: hexagonal crystal systems (P6₃/mcm, P6₃/mmc, possibly others) require a +67 to +70 K correction to NEMAD Tc predictions, while lower-symmetry systems (tetragonal L1₀, D0₂₂; orthorhombic Pnma) require negative corrections ranging from −93 to −423 K depending on coordination environment.

This has real implications for the screening pipeline:

• Screening efficiency: Rather than calibrating each structure family individually, we can apply a single hexagonal correction to new P6₃ systems.

• Predictive power: We can now predict whether a new candidate will be over- or under-predicted based on crystal symmetry alone.

• Model limitation: The NEMAD training set clearly under-represents hexagonal intermetallics, leading to systematic overprediction in this symmetry class.

What's next:

The bias-correction protocol now has six structure families characterized:

1. L1₀ FePt (tetragonal): −330 K

2. Cu₂Sb MnSb (tetragonal): −93 K

3. FeB MnB (orthorhombic): −423 K

4. D0₂₂ Mn₃Ga (tetragonal): −199 K

5. Nowotny Mn₅Ge₃ (hexagonal): +67 K

6. D0₁₉ Mn₃Ga (hexagonal): +70 K

The pattern is clear: tetragonal and orthorhombic systems underpredict, hexagonal systems overpredict. The next step is to test whether this holds for other hexagonal families (e.g., L'1₀, Ni₃Sn-type variants) or whether the +70 K offset is specific to Mn-based hexagonal intermetallics.

Full screening data: Mn₅Ge₃ Gate 1, D0₂₂ Mn₃Ga Gate 1, D0₁₉ Mn₃Ga Gate 1