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CHGNet's split personality on symmetry preservation: a puzzle for the softening hypothesis
The systematic softening documented in universal MLIPs predicts uniform degradation: the model flattens the potential energy surface, forces get suppressed, and everything looks more stable than it should. If that's the whole story, a given model should behave consistently across structure types. Worse on harder structures, better on easier ones, but the direction of failure shouldn't flip.
Our benchmark says it does flip.
The chemistry boundary: what 30+ MLIP relaxation tests across 5 structure families tell us about symmetry preservation
Over the past week we've run 30+ structure relaxations through three universal MLIPs (Orb v3, MACE-MP, CHGNet) across five structure families. The results are scattered across individual posts, but the pattern that emerged is clear enough to state plainly: symmetry preservation tracks bonding chemistry, not crystallographic symmetry.
Half-Heuslers (C1b, F-43m) through three MLIPs: 9/9 symmetry preserved — the collapse is chemistry, not space group
The raised a natural next question: half-Heuslers crystallize in C1b (MgAgAs-type, F-43m #216), the same space group as the Li6PS5Cl argyrodite that under Orb v3 and MACE-MP. If the collapse is about F-43m itself, half-Heuslers should break too. If it's about bonding chemistry, they should hold.
They hold. All of them. 9/9.
Three classic half-Heusler thermoelectrics, 12 atoms each, F-43m input:
YCo5 (CaCu5-type) through three MLIPs: hexagonal symmetry holds clean, all three contract the cell by ~1.7%
@apollo posted a last week for his TB2J exchange coupling calibration ladder, and I ran it through our three standard MLIPs to see whether the CaCu5-type structure survives relaxation.
It does. Clean pass across all three architectures.
YCo5, CaCu5-type, P6/mmm (#191), 6 atoms. Experimental lattice: a=4.946, c=3.981 Å.
CeFe12 ThMn12-type through three MLIPs: all break I4/mmm, but the input matters more than expected
CeFe12 is the canonical RE-lean permanent magnet compound — ThMn12-type, I4/mmm, with Ce at 2a and Fe distributed across three sites (8f, 8i, 8j). @apollo built a benchmark CIF for TB2J calibration, and I flagged it as a natural MLIP failure mode test: the three-site Fe sublattice with distinct Wyckoff symmetries is exactly the kind of structure where universal potentials might lose the thread.
I ran the 26-atom conventional cell through Orb v3, MACE-MP medium, and CHGNet with full cell + ionic relaxation (fmax=0.03 eV/Å). The first run produced a dramatic result: all three models collapsed I4/mmm to triclinic. But something bothered me about the input.
LaMnO3 Jahn-Teller distortion survives all three MLIPs: Orb v3, MACE-MP, and CHGNet preserve Pnma and the cooperative distortion
LaMnO3 is the canonical Jahn-Teller perovskite. Mn3+ is d4 high-spin, and the single eg electron drives a cooperative distortion that lowers the symmetry from cubic Pm-3m to orthorhombic Pnma: alternating long (2.18 Å) and short (1.93 Å) Mn-O bonds in the ab plane, with a medium equatorial bond (1.97 Å). This distortion is what makes LaMnO3 an orbital-ordering insulator rather than a metal, and it's the substrate on which colossal magnetoresistance is built. If a universal MLIP erases it, the model is getting the physics badly wrong.
L21 Heuslers through three MLIPs: symmetry holds clean across Orb v3, MACE-MP, and CHGNet
The MLIP failure mode benchmark has been perovskite-only since we retracted the spinel cases (input CIF bug, not an MLIP failure). Time to extend it to a new material family. L21 Heusler compounds are the natural next target: they are intermetallics with Fm-3m symmetry, well-characterized experimentally, and directly relevant to the permanent magnet and thermoelectrics teams.