Fix floating point imprecision error in ordering property of CollinearMagneticStructureAnalyzer (#3574)

* Modified ordering property of CollinearMagneticStructureAnalyzer to account for floating point imprecision, added test for that.

* improve CollinearMagneticStructureAnalyzer.ordering doc str, drop isclose in favor abs(tot_mag) > 1e-8

* compress tests/files/magnetic.example.CuO.mcif

* add comment with source DOI for mcif file

* link PR that added CuO AFM test

---------

Co-authored-by: kueltzen <[email protected]>
Co-authored-by: Janosh Riebesell <[email protected]>

Author: kaueltzen

pymatgen/analysis/magnetism/analyzer.py

@@ -481,10 +481,15 @@ def number_of_unique_magnetic_sites(self, symprec: float = 1e-3, angle_tolerance
     @property
     def ordering(self) -> Ordering:
         """Applies heuristics to return a magnetic ordering for a collinear
-        magnetic structure. Result is not guaranteed for correctness.
+        magnetic structure. Result is not guaranteed to be correct, just a best
+        guess. Tolerance for minimum total magnetization to be considered
+        ferro/ferrimagnetic is 1e-8.
 
         Returns:
-            Ordering: Enum ('FiM' is used as the abbreviation for ferrimagnetic)
+            Ordering: Enum  with values FM: ferromagnetic, FiM: ferrimagnetic,
+                AFM: antiferromagnetic, NM: non-magnetic or Unknown. Unknown is
+                returned if magnetic moments are not defined or structure is not collinear
+                (in which case a warning is issued).
         """
         if not self.is_collinear:
             warnings.warn("Detecting ordering in non-collinear structures not yet implemented.")
@@ -503,9 +508,9 @@ def ordering(self) -> Ordering:
 
         is_potentially_ferromagnetic = np.all(magmoms >= 0) or np.all(magmoms <= 0)
 
-        if total_magnetization > 0 and is_potentially_ferromagnetic:
+        if abs(total_magnetization) > 1e-8 and is_potentially_ferromagnetic:
             return Ordering.FM
-        if total_magnetization > 0:
+        if abs(total_magnetization) > 1e-8:
             return Ordering.FiM
         if max_magmom > 0:
             return Ordering.AFM

tests/analysis/magnetism/test_analyzer.py

@@ -34,6 +34,10 @@ def setUp(self):
 
         self.NiO_expt = Structure.from_file(f"{TEST_FILES_DIR}/magnetic.example.NiO.mcif", primitive=True)
 
+        # CuO.mcif sourced from https://www.cryst.ehu.es/magndata/index.php?index=1.62
+        # doi: 10.1088/0022-3719/21/15/023
+        self.CuO_expt = Structure.from_file(f"{TEST_FILES_DIR}/magnetic.example.CuO.mcif.gz", primitive=True)
+
         lattice = Lattice.cubic(4.17)
         species = ["Ni", "O"]
         coords = [[0, 0, 0], [0.5, 0.5, 0.5]]
@@ -161,28 +165,32 @@ def test_get_ferromagnetic_structure(self):
         assert CollinearMagneticStructureAnalyzer(s1).matches_ordering(s2_prim)
 
     def test_magnetic_properties(self):
-        msa = CollinearMagneticStructureAnalyzer(self.GdB4)
-        assert not msa.is_collinear
+        mag_struct_analyzer = CollinearMagneticStructureAnalyzer(self.GdB4)
+        assert not mag_struct_analyzer.is_collinear
 
-        msa = CollinearMagneticStructureAnalyzer(self.Fe)
-        assert not msa.is_magnetic
+        mag_struct_analyzer = CollinearMagneticStructureAnalyzer(self.Fe)
+        assert not mag_struct_analyzer.is_magnetic
 
         self.Fe.add_site_property("magmom", [5])
 
-        msa = CollinearMagneticStructureAnalyzer(self.Fe)
-        assert msa.is_magnetic
-        assert msa.is_collinear
-        assert msa.ordering == Ordering.FM
+        mag_struct_analyzer = CollinearMagneticStructureAnalyzer(self.Fe)
+        assert mag_struct_analyzer.is_magnetic
+        assert mag_struct_analyzer.is_collinear
+        assert mag_struct_analyzer.ordering == Ordering.FM
 
-        msa = CollinearMagneticStructureAnalyzer(
+        mag_struct_analyzer = CollinearMagneticStructureAnalyzer(
             self.NiO,
             make_primitive=False,
             overwrite_magmom_mode="replace_all_if_undefined",
         )
-        assert msa.number_of_magnetic_sites == 4
-        assert msa.number_of_unique_magnetic_sites() == 1
-        assert msa.types_of_magnetic_species == (Element.Ni,)
-        assert msa.get_exchange_group_info() == ("Fm-3m", 225)
+        assert mag_struct_analyzer.number_of_magnetic_sites == 4
+        assert mag_struct_analyzer.number_of_unique_magnetic_sites() == 1
+        assert mag_struct_analyzer.types_of_magnetic_species == (Element.Ni,)
+        assert mag_struct_analyzer.get_exchange_group_info() == ("Fm-3m", 225)
+
+        # https://github.com/materialsproject/pymatgen/pull/3574
+        mag_struct_analyzer = CollinearMagneticStructureAnalyzer(self.CuO_expt)
+        assert mag_struct_analyzer.ordering == Ordering.AFM
 
     def test_str(self):
         msa = CollinearMagneticStructureAnalyzer(self.NiO_AFM_001)