I tried to recompute total magnetization for some materials, e.g. FeO (mp-1178232). All input files and parameters were set by pymatgen in automation. The structure was directly got from MP.
After a few ionic steps, the relaxation was done (not surprised) but the total magnetization I obtained is 8 magneton Bohr/cell instead of 4 like in MP.
I wonder that the total magnetization stored in MP is normalized to the reduced formula or there is something wrong here.
I am only a user of the MP, not a developer, but I strongly suspect that you are right in your observation that this is a issue of unit cells. If you download the VASP input files, it will give you a conventional unit cell with 8 atoms in total. On the other hand the data given on the MP site clearly indicates a smaller unit cell with only 4 atoms and different lattice vectors.
If you use MPRester to get the structure instead of importing it from the POSCAR file, you will get the reduced cell with only 4 atoms.
with MPRester() as mpr:
struct = mpr.get_structure_by_material_id("mp-1178232")
results in
Structure Summary
Lattice
abc : 3.1664873178599873 3.1107873647088145 6.192646264665004
angles : 60.08497835710322 59.24753083625531 59.40592016569469
volume : 42.71541095355241
A : -0.0 2.23904465 2.23904466
B : 2.17624058 0.01627279 2.22277083
C : 4.32960659 4.42727827 0.05080999
PeriodicSite: Fe (0.0000, 0.0000, 0.0000) [-0.0000, 0.0000, -0.0000]
PeriodicSite: Fe (2.1648, 2.2136, 0.0254) [-0.0000, 0.0000, 0.5000]
PeriodicSite: O (2.0880, 2.2873, 2.1908) [0.5190, 0.4571, 0.2525]
PeriodicSite: O (4.4179, 4.3953, 2.3218) [0.4810, 0.5429, 0.7475]
I just wanted to note that in the latest database release the mp-1178232 structure has now gone through our magnetic ordering workflow so we report it in an AFM configuration. This is the structure now returned by the API:
Full Formula (Fe4 O4)
Reduced Formula: FeO
abc : 3.154629 5.352971 5.349655
angles: 79.868209 90.553757 73.688429
Sites (8)
# SP a b c magmom
--- ---- -------- -------- -------- --------
0 Fe 0.495784 0.000992 0.500626 -3.746
1 Fe 0.75123 0.500375 0.250754 -3.75
2 Fe 0.00268 0.998702 0.999844 3.746
3 Fe 0.249611 0.500225 0.750896 3.751
4 O 0.872262 0.251921 0.643608 0.016
5 O 0.390034 0.250589 0.143084 -0.017
6 O 0.111627 0.749499 0.355918 -0.04
7 O 0.626772 0.747698 0.855272 0.04
As a result, the total magnetization for this material is now zero.