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Magnetism

Whether the calculation is spin-polarised, how the spins are arranged, and what to start the moments at.

No model yet

Quantity What it is Model
is_magnetic Whether the ground state is spin-polarised none
ordering Non-magnetic, ferromagnetic, antiferromagnetic, ferrimagnetic none
magnetic_moments Starting moment per site none

These are three questions, not one. The first decides whether to switch spin polarisation on; the second decides which arrangement to converge towards; the third is what actually goes into the input file.

What a model would have to beat

Core answers the first question heuristically: if the structure contains a magnetic candidate element, spin polarisation is switched on, otherwise it is not. That rule is the baseline, and it is the number a model has to improve on — not a majority-class baseline.

Nothing answers the other two.

Open before a model can exist

  • A target contract, per quantity. "Magnetic" needs pinning to something checkable, and an ordering label needs the analysis that produced it named.
  • A dataset. Materials Project carries both a computed total moment and an ordering label.
  • Somewhere on Core's side for the second and third answers. The calculation hints carry a single boolean today.
  • A prediction type that survives leaving the scalars behind. An ordering label is a string and fits; per-site moments are one value per site, and a contract would have to fix the site order for them to mean anything.

Two things that make this harder than metallicity

Switching spin polarisation on is not the same as making the calculation magnetic. Core emits nspin = 2 and no starting_magnetization, which in Quantum ESPRESSO starts every species at zero moment: the two spin channels are identical and the run usually settles back to the non-magnetic solution. So is_magnetic on its own does not buy a magnetic calculation. magnetic_moments is the quantity that does.

It is worse on the non-collinear path. When spin-orbit coupling is enabled Core writes noncolin and lspinorb and drops the magnetism advice entirely, so the case spin-orbit exists for -- a magnetic heavy-element system -- gets the most expensive spin treatment available and no magnetism at all. A non-collinear run needs directions as well as magnitudes, which is a fourth thing a model could be asked for and a reason to settle the shape of this advice before fitting anything to it.

An antiferromagnetic ordering may not fit in the chemical cell. Converging one can require a magnetic supercell, which changes the structure every later recommendation is made for — including the k-point mesh. Magnetism is the first setting whose answer can feed backwards into the pipeline rather than only forwards into the input file, and where it sits in the ordering has to be settled before a model is wired in.