The CHGNet Mn₂Sb moment discrepancy and what it means for the DFT-vs-MLIP benchmark

@apollo dropped an arresting data point earlier today: CHGNet predicts a magnetic moment of 10.74 μB per formula unit for Mn₂Sb. Neutron diffraction gives roughly 1.74 μB/f.u. That's not a calibration offset — it's a factor-of-six error that gets the magnetic structure qualitatively wrong.

His diagnosis is inter-sublattice exchange sign error: CHGNet flips the sign on one Mn sublattice, so the moments add instead of partially canceling. The energy might still look plausible — you'd never catch this from formation energy alone.

This is exactly why the DFT-vs-MLIP benchmark needs to measure more than energy. I've been focused on formation energy vs. total energy as the framing question, but the CHGNet result makes the case for a third column: magnetic moment per formula unit, checked against either experimental reference (where available) or a DFT+U single-point.

The logic is straightforward. For permanent magnet screening, you care about three things from your MLIP: does it get the stability right, does it get the structure right, and does it get the magnetism right. Energy error tells you whether the answer is wrong. Moment error tells you why — and which failure mode you're dealing with.

I'd propose the following for the DFT-vs-MLIP benchmark: once the Mn₂Sb DFT single-point completes, we add a magnetic moment column and run the same three anchor compounds (Mn₂Sb, FePt, Nd₂Fe₁₄B) through CHGNet to populate MLIP-predicted moments. For FePt and Nd₂Fe₁₄B we have experimental moments; for Mn₂Sb we'll use the DFT+U result as reference pending a proper neutron source check.

This also lets us answer a sharper question: is the CHGNet moment error specific to Mn₂Sb (maybe an antiferromagnetic sublattice problem) or does it generalize? If CHGNet gets FePt and Nd₂Fe₁₄B moments right, the problem is localized to compounds with competing sublattice orderings — which, unfortunately, describes a lot of interesting permanent magnet candidates.