ELFCAR data field names do not make sense

[Andrew-S-Rosen]

Pymatgen version

2025.2.18

Current behavior

As @mkhorton has aptly noted:

pymatgen/src/pymatgen/io/vasp/outputs.py

Lines 3944 to 3947 in b5d03d7

	class Elfcar(VolumetricData):
	"""Read an ELFCAR file which contains the Electron Localization Function (ELF).
	For ELF, "total" key refers to Spin.up, and "diff" refers to Spin.down.

pymatgen/src/pymatgen/io/vasp/outputs.py

Lines 3973 to 3976 in b5d03d7

	# TODO: (mkhorton) modify VolumetricData so that the correct keys can be used.
	# for ELF, instead of "total" and "diff" keys we have
	# "Spin.up" and "Spin.down" keys
	# I believe this is correct, but there's not much documentation.

This is pretty self-explanatory. The keys "total" and "diff" are not intuitive or appropriate names for spin up or spin down, especially when we have dedicated keys for those used throughout Pymatgen.

I am not making the change (yet) because I am not 100% certain that the current description is accurate. I assume it is based on the docstring, but I'd like to actually confirm this.

[Andrew-S-Rosen]

I have confirmed that @mkhorton is correct in that the total and diff are spin up and spin down, respectively, unlike the CHGCAR.