How to set atoms.info["spin"] and atoms.info["charge"] for MACE-POLAR-1

[jihuh]

Hello,

I am trying to run MACE-POLAR-1 for an organic molecular liquid.
My system has zero total charge, and all molecules are closed shell.

The guidelines (https://mace-docs.readthedocs.io/en/latest/guide/polar_mace.html) seem to suggest that I should set atoms.info,
but I could not find a description as to what these mean.

1. Is it mandatory that I set atoms.info["charge"] and atoms.info["spin"] ?
   Simulations seem to run without setting them. In this case, what are their default values?
2. For a system consisted of closed shell molecules only, should atoms.info["spin"] =0 (total number of unpaired electrons) or 1(spin multiplicity 2S+1)?

from mace.calculators import mace_polar

calc = mace_polar(
    model="polar-1-m",
    device="cpu",           # or "cuda"
    default_dtype="float64" # use float32 for faster MD
)

atoms.info["charge"] = 0
atoms.info["spin"] = 1
atoms.info["external_field"] = [0.0, 0.0, 0.0]
atoms.calc = calc

Thank you.

[jihuh]

These are the answers I got from asking AI (April 6, 2026)
Hope this helps anyone else with the same question.

If you do not set them, mace_polar defaults to:

atoms.info["charge"] -> 0.0
atoms.info["spin"] -> 1.0
atoms.info["external_field"] -> [0.0, 0.0, 0.0]
That is why the calculation still runs.

The path is:

Missing ASE info keys are first read as None in mace/data/utils.py (line 197)
They are then replaced with defaults in mace/data/atomic_data.py (line 336), mace/data/atomic_data.py (line 352), and mace/data/atomic_data.py (line 382)
So omitting those fields is effectively the same as:

atoms.info["charge"] = 0
atoms.info["spin"] = 1
atoms.info["external_field"] = [0.0, 0.0, 0.0]
One detail: internally PolarMACE uses total_spin - 1 in its charge/spin partitioning, see mace/modules/extensions.py (line 752). So spin=1 is the neutral default baseline it expects.