Understanding Structures and Properties in the Materials Project

1. Why Structures May Look Different

When retrieving structures from the new Materials Project (MP) API, you may notice that the lattice parameters and angles do not match the conventional or primitive unit cells you might expect from textbooks or the legacy MP database.

Key points:

• Non-unique representations: A crystal can be represented in many valid ways. The MP database does not store or return the primitive or conventional unit cell by default.

• Possible supercells: The stored cell may contain multiple repeat units in a "non-standard" representation.

• Origin of structures: Structures often come from experimental sources (e.g., ICSD) or are generated by substitutions on known prototypes, and are typically not symmetry-reduced before calculations.

• Why not always use symmetry reduction? Automatic numeric symmetry detection can occasionally misidentify a material’s symmetry, leading to incorrect primitive cells.

If you need a conventional or primitive cell, you can convert the returned structure in Python:

from mp_api.client import MPRester

with MPRester("YOUR_API_KEY") as mpr:
    docs = mpr.materials.summary.search(material_ids=["mp-13", "mp-90"])
    structures = {str(doc.material_id): doc.structure for doc in docs}

# Convert to conventional cells
conventional = {mid: s.to_conventional() for mid, s in structures.items()}

# Convert to primitive cells
primitive = {mid: s.get_primitive_structure() for mid, s in conventional.items()}

2. Changes in Magnetic Moments and Other Properties

You may also notice differences in computed properties (e.g., magnetic moments, volumes) between the legacy MP database and the new one.

Reason:

• Legacy MP calculations were performed with the PBE GGA functional.

• New MP calculations use the r²SCAN meta-GGA functional, which is a higher-level, generally more accurate theory for many material properties.

Impact of r²SCAN:

• Improves accuracy for:

  • Magnetic moments in many oxides

  • Magnetic ordering in antiferromagnets

  • Thermodynamic stability

• Known tendency:

  • Overestimates on-site ferromagnetic moments in elemental metals (e.g., Fe, Ni) compared to experiment and PBE

3. Summary Table

Aspect	Legacy MP	Newer MP data
DFT functional	PBE (GGA) and GGA+U	r²SCAN (meta-GGA)
Returned cell type	Often conventional cell	Arbitrary non-standard cell (can convert)
Magnetic moments in metals	Closer to experiment	Often overestimated in transition metals
Thermodynamic accuracy	Good	Improved
Magnetic ordering in oxides	Sometimes incorrect	Improved

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4. Best Practices

• Always convert to a primitive or conventional cell if you need a standard representation. The Materials Project uses pymatgen and spglib to determine primitive and conventional cells

• Be aware of functional differences when comparing old and new MP data.

• Treat very small deviations (e.g., ~1e-16 in coordinates) as numerical noise.

• Consult the Materials Project API documentation for updates — this behavior will be documented more clearly in the future.