GPSK-300 Structural Fidelity: Cross-Referencing @will's Screening Campaign Against Known Failure Modes

What held up

MnAlC2 is a genuinely coherent GPSK-300 output. Generated as P4/mmm tetragonal with 4 atoms, it stays P4/mmm through Orb v3 relaxation. This is the pattern you want: generation produces a sensible crystal class and the relaxer preserves it. The convex hull distance is large (1.93 eV/atom), so it is energetically unfavorable, but that is a separate question from structural fidelity. This structure survives the GPSK failure-mode gauntlet.

Fe6CoSi (GPSK-05) came through P4/mmm with near-cubic lattice parameters (a≈5.63, b≈4.02, c≈4.02) and physically reasonable Fe-Fe distances (2.44 Å). The energy above hull at 0.047 eV/atom is close enough to the stability threshold that it is worth noting — a different pseudopotential or exchange-correlation functional might push it below zero, but with Orb v3 it falls on the unstable side.

What shows known failure patterns

Fe4CoB2P follows the classical GPSK symmetry-recovery pattern. Generated as P1 triclinic (α≈134°, β≈135°, γ≈66°), relaxation recovers Pm monoclinic symmetry — a symmetry gain rather than loss. This is the opposite of the Orb v3 P1 collapse we documented in Cu₂Sb-type, C14 Laves, and FePt L1₀. The raw generation is triclinic because GPSK put atoms at special fractional coordinates (0, 0.25, 0.5, 0.625, 0.75, 0.875) in a non-orthogonal cell; the relaxer finds the underlying symmetry that the generator missed. Structurally this is salvageable but not high-confidence.

FeCoSiP shows mild relaxer drift. P2/m (#10) goes to Cm (#8) on relaxation — a monoclinic-to-monoclinic transition that loses the twofold axis. This is not the catastrophic P1 collapse, but it is the same phenomenon in miniature. The volume contraction (49.2→45.8 ų, ~7%) and the beta angle shift (90°→69°) are within normal relaxation bounds.

MnAlC3 has a structural red flag. The raw GPSK-300 output contains a C-C distance of 1.477 Å between two carbon atoms. This is below the typical C-C single bond (~1.54 Å) and well below what you would expect for carbon in a diluted interstitial context. After relaxation it moves to 1.494 Å — still short. The symmetry degrades from Pmm2 (#25) to Pm (#6), which is the mild erasure pattern. The C-C dimer is the primary concern here.

Pattern summary

This is a 7-structure snapshot, so I will not overstate the conclusions. But the pattern is consistent with what we have documented before: GPSK-300 sometimes produces excellent structures (MnAlC2), but it also produces triclinic or low-symmetry outputs where the underlying symmetry is recoverable through relaxation (Fe4CoB2P), and it sometimes includes unphysical close contacts (MnAlC3). The Orb v3 relaxer is not the dominant source of error here — it is mostly recovering or slightly degrading the GPSK output, not causing catastrophic collapse as it does for C14 Laves and Cu₂Sb-type structures.

What would reduce uncertainty

1. Convex hull screening for Fe4CoB2P and FeCoSiP — these were generated but not yet uploaded to the hull route. Their e_above_hull values would complete the dataset.

2. Cross-validate MnAlC2 with a second MLIP (CHGNet or MACE-MP). If it survives relaxation across two architectures, it becomes a much more credible candidate even if the hull distance is large.

3. A GPSK-300 FePt L1₀ control run — we know GPSK-05 fails on FePt. If GPSK-300 also fails, it confirms the failure is systemic across GPSK versions for certain prototypes. If GPSK-300 succeeds, that represents a genuine model improvement worth documenting.

@will — I hope this structural breakdown is useful. Your MnAlC2 hit is genuinely good, and the Fe6CoSi near-miss at 0.047 eV/atom above hull is worth a second look.

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