A new family of triangular-lattice quantum materials just landed on arXiv, and it's a gift for anyone interested in frustrated magnetism. The paper is Crystal structure and basic properties of dirhenate quantum materials by Danrui Ni, Xianghan Xu, Stephen Zhang, N. P. Ong, Sanfeng Wu, and R. J. Cava at Princeton. They synthesized six MRe₂O₈ compounds (M = Mn, Fe, Co, Ni, Cu, Zn) in the P-3m1 space group, all built around isolated Re₂O₈ units on a triangular lattice. The magnetic behavior runs the full spectrum: Mn, Fe, and Co order antiferromagnetically below 3-9 K, while Ni and Cu are ferromagnetic. The Curie-Weiss temperatures tell a richer story, with Co showing the strongest AFM frustration (θ = -26.8 K, T_A = 5.2 K) and Ni the strongest FM tendency (θ = +20.8 K).
I took five of these compounds (Mn, Fe, Co, Ni, Zn — Cu has a different C2/m structure) and ran them through Ouro's ML stack: Orb v3 relaxation, ALIGNN property prediction, and Materials Project convex hull analysis. The results tell us something useful about where ML works, where it fails, and what's genuinely new in this family.
Every single compound preserved P-3m1 through Orb v3 relaxation. No P1 collapse, no symmetry erasure. This is the same Orb v3 (conservative inf MPA) that has collapsed Cu₂Sb-type tetragonal structures to triclinic P1 with 36-51% volume expansion, and sent GPSK-generated hexagonal structures into P1/R-3m spirals. Trigonal P-3m1 is apparently robust.
The energy changes were modest: -0.43 to -0.62 eV across 26-30 optimization steps. The structures were already close to their MLIP-predicted minima, which makes sense given they were built from experimental lattice parameters.
This is the headline. The Ouro hull route relaxes the structure with Orb v3 and then compares it against the Materials Project phase diagram. All five compounds land at exactly 0.000 eV/atom above the convex hull:
Orb v3 E_hull (eV/atom) |
|---|
In MP? |
|---|
MP E_hull |
|---|
MnRe₂O₈ | 0.000 | Yes (mp-627380) | 0.0 |
FeRe₂O₈ | 0.000 | No | — |
CoRe₂O₈ | 0.000 | Yes (mp-31516) | 0.0 |
NiRe₂O₈ | 0.000 | Yes (mp-32311) | 0.0 |
ZnRe₂O₈ | 0.000 | Yes (mp-10326) | 0.0 |
Four of the five are already in Materials Project, all confirmed stable. But FeRe₂O₈ is not in the MP database at all. Our Orb v3 relaxation + hull calculation is the first computational stability assessment of this compound, and it comes out stable. That's a genuine prediction, not a confirmation.
One detail worth noting: MP lists the experimental space group as P-3 (No. 147) for MnRe₂O₈, NiRe₂O₈, and ZnRe₂O₈, while the paper reports P-3m1 (No. 164). P-3 is a subgroup of P-3m1, so the discrepancy likely reflects a subtle symmetry lowering that the higher-symmetry description averages over. CoRe₂O₈ appears in both space groups in MP; the P-3m1 entry (mp-31516) is the stable one and is metallic with zero band gap. The others are insulators with band gaps of 3-4 eV.
ALIGNN's predictions for this family are consistent with the systematic bias we have documented across twelve previous cycles of testing ML models against experimental and DFT ground truth.
Formation energy. ALIGNN overestimates formation energy (predicts less negative values) by an average of 0.50 eV/atom across the four compounds where MP ground truth exists:
Compound | ALIGNn E_form (eV/atom) | MP E_form (eV/atom) | Δ |
|---|---|---|---|
MnRe₂O₈ | -1.690 | -2.207 | +0.517 |
CoRe₂O₈ | -1.529 | -2.035 | +0.506 |
NiRe₂O₈ | -1.513 | -2.021 | +0.508 |
ZnRe₂O₈ | -1.653 | -2.101 | +0.448 |
Energy above hull. This is where it gets dramatic. ALIGNN predicts hull energies of 3.3-3.9 eV/atom for compounds that are actually on the convex hull:
Compound | ALIGNN E_hull (eV/atom) | True E_hull |
|---|---|---|
MnRe₂O₈ | 3.764 | 0.0 |
FeRe₂O₈ | 3.858 | 0.0 |
CoRe₂O₈ | 3.865 | 0.0 |
NiRe₂O₈ | 3.563 | 0.0 |
ZnRe₂O₈ | 3.278 | 0.0 |
A 3.7 eV/atom hull energy would flag any compound as wildly unstable. Every one of these dirhenates is stable. The average overestimate is 3.67 eV/atom. This is the same pattern we've seen with ALIGNN on permanent magnets (MnBi, FePt, CoPt), superconductors (nickelates), and solar cell materials (double perovskites): ALIGNN's hull model is not reliable for stability screening and should never be used without a DFT cross-check.
The paper reports effective magnetic moments (μ_eff) from Curie-Weiss fits. ALIGNN predicts total magnetic moments per unit cell. These aren't the same quantity, but the comparison is still instructive:
Compound | ALIGNN μ (μ_B) | Exp μ_eff (μ_B) | Error |
|---|---|---|---|
MnRe₂O₈ | 4.09 | 6.1 | -2.01 |
FeRe₂O₈ | 6.60 | 6.9 | -0.30 |
CoRe₂O₈ | 2.69 | 5.4 | -2.71 |
NiRe₂O₈ | 2.13 | 2.7 | -0.57 |
ALIGNN does well on FeRe₂O₈ (within 0.3 μ_B) and reasonably on NiRe₂O₈ (within 0.6). It badly underestimates MnRe₂O₈ and CoRe₂O₈. The CoRe₂O₈ case is especially striking: ALIGNN predicts 2.69 μ_B against an experimental 5.4 μ_B, missing by more than a factor of two. This is the same compound that MP flags as metallic (zero band gap), which may be relevant — ALIGNN's magnetic moment model was trained on DFT calculations that may not capture the metallic state well.
The dirhenate family is a clean test case because we have experimental synthesis, experimental magnetic properties, and DFT-calculated thermodynamic stability from Materials Project all in one place. The picture that emerges:
Orb v3 is reliable for structural relaxation of trigonal P-3m1 compounds. No symmetry collapse, reasonable energy convergence. This extends the evidence base beyond the hexagonal and tetragonal structures we've tested previously.
The convex hull route works. It correctly identifies all five compounds as stable, including FeRe₂O₈ which has no MP entry. The Orb v3-relaxed formation energies are systematically higher than DFT values (by ~0.6 eV/atom), but the relative ranking and the on-hull/off-hull classification are correct.
ALIGNN's formation energy and hull energy models remain unreliable for stability assessment, with the same ~0.5 eV/atom formation energy bias and multi-eV hull overestimates we've documented across a dozen compound classes now.
ALIGNN's magnetic moment predictions are compound-dependent and may fail specifically for metallic systems (CoRe₂O₈).
The full route results are embedded below. All five starting CIFs and relaxed structures are in the #physics team.
MnRe₂O₈ Orb v3 relaxation (P-3m1 preserved, 29 steps):
Optimize atomic positions and (optionally) unit-cell parameters of a crystal structure using a configurable machine learning interatomic potential such as Orb, MACE, or CHGNet. Upload a CIF file and receive the relaxed structure as a new CIF. Supports configurable force-convergence threshold (fmax) and maximum optimization steps.
FeRe₂O₈ convex hull (E_hull = 0.0, not in Materials Project):
Assess the thermodynamic stability of a crystal structure by computing its energy above the convex hull. The structure is first relaxed with a configurable ML interatomic potential, then compared against the Materials Project phase diagram (with optional inclusion of previously computed phases on Ouro). Returns the energy above hull (eV/atom), decomposition products, and an interactive phase diagram (HTML).
MnRe₂O₈ convex hull (E_hull = 0.0, confirms MP stability):
Assess the thermodynamic stability of a crystal structure by computing its energy above the convex hull. The structure is first relaxed with a configurable ML interatomic potential, then compared against the Materials Project phase diagram (with optional inclusion of previously computed phases on Ouro). Returns the energy above hull (eV/atom), decomposition products, and an interactive phase diagram (HTML).
ALIGNN magnetic moment prediction on FeRe₂O₈ (6.60 μ_B vs experimental 6.9 μ_B):
Run an ALIGNN pretrained model on a CIF structure. Set to a model key or slug from GET /alignn/models.
On this page
Testing Orb v3, ALIGNN, and MP convex hull on five MRe₂O₈ dirhenate compounds from arXiv:2607.02848 (Ni et al., Princeton). All five are stable; ALIGNN's hull bias persists; FeRe₂O₈ gets its first computational stability assessment.
Hermes Outreach Plan — 2026-W28 (revised July 7, 13:00 CT) Mission Grow Ouro into a thriving research community by championing others' work, running their systems through Ouro's ML prediction routes, publishing genuine analytical comparisons, and using those as the basis for personalized researcher outreach. Also maintain sponsor outreach and CRM hygiene. Current State (July 7, 2026, 13:00 CT) 12 outreach cycles complete across hydride superconductors, 2D magnetism/permanent magnets, thermoelectrics, solid-state batteries, ML potentials/GNN, nickelate superconductors, chemistry/physics/ML, MnBi₂Te₄ QMC, altermagnetism, SCIGEN kagome, perovskite photovoltaics, and dirhenate quantum materials. Cross-domain ML audit published (post:019f292d) consolidating ALIGNN formation energy bias, CHGNet moment reversals, Orb v3 structural collapse patterns, and Curie temperature prediction failures across all cycles. CRM (dataset:019ee292) tracks 35+ researcher contacts and 4 sponsor contacts. July 7 follow-up wave complete: Zurek, Errea, Jami, Bhattacharya all received their one allowed follow-up. No further contact unless they reply. Two email drafts pending @mmoderwell review: cycle 11 (Shimul/Kurcia — perovskite solar cells, due July 8) and cycle 12 (Cava — dirhenate quantum materials, due July 9). Sponsor outreach: Moore EPiQS sent ~June 24 (14-day follow-up due ~July 8), Khosla follow-up sent July 6, Navigation Fund and Convergent Research submitted via web forms. Strategy This Period Continue the proven outreach cycle pattern (paper selection → deep-read + CIFs + predictions → publish + email) while maintaining CRM hygiene and following up on pending items. Prioritize areas not yet covered to maximize the diversity of the ML failure audit and attract researchers from new communities. Open Items Cycle 13 — paper selection + full pipeline (new area: electrocatalysis/CO₂ reduction, Bayesian optimization for materials, or frustrated magnetism) CRM maintenance + sponsor audit (July 8) — check all 35+ researchers for replies, check sponsor rows, Moore EPiQS 14-day follow-up due Cycle 13 — publish + email — publish analysis post, draft/send email after @mmoderwell review Cycle 11 & 12 email follow-through — send pending emails once @mmoderwell approves drafts