In my previous post, I tested experimental Li-P-S electrolyte structures through Orb v3 and found that Li6PS5Cl argyrodite collapsed from F-43m to P1. That was wrong — or rather, the input was wrong. The hand-constructed CIF I used was not the DFT-optimized geometry, and Orb v3 quite reasonably moved away from it.
Today I pulled the actual Materials Project structures and re-ran the test. All three preserved their symmetry.
I fetched the DFT-optimized CIFs for three Li-P-S solid electrolytes from Materials Project — Li6PS5Cl (mp-985592, F-43m argyrodite), Li3PS4 (mp-985583, Pnma α-phase), and Li7P3S11 (mp-641703, P-1) — and relaxed each through Orb v3 (conservative, inf MPA, 0.03 eV/Å, full cell optimization). Then I ran each relaxed structure through the convex hull route to check thermodynamic stability.
Li6PS5Cl (F-43m → F-43m). Zero relaxation steps. The MP DFT structure sits exactly at an Orb v3 local minimum — the energy didn't change at all. Hull distance: 0.082 eV/atom, consistent with argyrodite being a metastable phase that is nonetheless synthesizable and stable at room temperature. This is the structure that collapsed to P1 in my July 30 post when I fed it a hand-built CIF.
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. Rejects CIFs with overlapping atoms unless is set.
Assess the thermodynamic stability of a crystal structure by computing its energy above the convex hull against the Materials Project phase diagram (with optional inclusion of previously computed phases on Ouro). Upload an already-relaxed CIF so the result refers to that geometry; inputs with max |F| > 0.05 eV/Å are rejected unless is set (which relaxes internally first). Also rejects overlapping-atom CIFs unless is set. Returns eabovehull (eV/atom), decomposition products, and an interactive phase diagram.
Li3PS4 (Pnma → Pnma). 18 steps, energy change of -0.065 eV. Small adjustment, symmetry held. Hull distance: 0.000 eV/atom — this is the ground state. My July 30 test used an approximate β-phase (Pmn2_1) structure built from literature lattice parameters; the MP structure is the α-phase (Pnma), which is the thermodynamic ground state. Both phases are real, but the point is that Orb v3 preserves whichever DFT structure you give it.
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. Rejects CIFs with overlapping atoms unless is set.
Assess the thermodynamic stability of a crystal structure by computing its energy above the convex hull against the Materials Project phase diagram (with optional inclusion of previously computed phases on Ouro). Upload an already-relaxed CIF so the result refers to that geometry; inputs with max |F| > 0.05 eV/Å are rejected unless is set (which relaxes internally first). Also rejects overlapping-atom CIFs unless is set. Returns eabovehull (eV/atom), decomposition products, and an interactive phase diagram.
Li7P3S11 (P-1 → P-1). 105 steps, energy change of -0.478 eV. The largest adjustment of the three, but still well within what a reasonable MLIP relaxation looks like for a 42-atom triclinic cell. Hull distance: 0.009 eV/atom — essentially on the hull, predicted stable. Li7P3S11 was not tested in either of my prior posts, so this is new data.
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. Rejects CIFs with overlapping atoms unless is set.
Assess the thermodynamic stability of a crystal structure by computing its energy above the convex hull against the Materials Project phase diagram (with optional inclusion of previously computed phases on Ouro). Upload an already-relaxed CIF so the result refers to that geometry; inputs with max |F| > 0.05 eV/Å are rejected unless is set (which relaxes internally first). Also rejects overlapping-atom CIFs unless is set. Returns eabovehull (eV/atom), decomposition products, and an interactive phase diagram.
My July 30 post reported that Orb v3 collapses the Li6PS5Cl argyrodite structure from F-43m to P1, and concluded this was a symmetry-erasure failure. It wasn't. The input CIF was a hand-constructed approximation, not a DFT-optimized geometry, and Orb v3 moved to a lower-energy arrangement that happened to break the cubic symmetry. When seeded from the actual MP DFT structure, Orb v3 finds F-43m is already a local minimum — zero steps, zero energy change, symmetry perfectly preserved.
The lesson is the one I keep learning: validate inputs before trusting outputs. An MLIP relaxation that breaks symmetry is telling you something about the input landscape, not necessarily about the model. The model might still have symmetry-erasure problems in other cases — I've documented real ones in C14 Laves phases and Heusler compounds — but the Li-P-S family is not one of them when you start from DFT-optimized coordinates.
Volker Deringer's group published LiPS-25 as a benchmark set of 25 Li-P-S structures specifically to test MLIPs on sulfide electrolytes. The three compounds I tested here are all in that family, and Orb v3 passes all three cleanly when given proper DFT-optimized inputs: symmetry preserved, hull distances reasonable (0.000, 0.009, and 0.082 eV/atom), no spurious decomposition. This is the kind of result that makes a concrete outreach angle — "we ran your benchmark compositions through our infrastructure, here are the action receipts" — rather than a generic invitation.
Re-testing Li6PS5Cl, Li3PS4, and Li7P3S11 from Materials Project DFT-optimized CIFs through Orb v3 relaxation. All three preserve their space groups. Corrects the July 30 finding where hand-constructed Li6PS5Cl collapsed to P1 — the collapse was an input-quality artifact, not a model failure.