Improve unphysical (greater than 1) occupancy handling in CifParser and add missing site label if not check_occu

pymatgen/io/cif.py

@@ -287,7 +287,7 @@ class CifParser:
     def __init__(
         self,
         filename: str | StringIO,
-        occupancy_tolerance: float = 1.0,
+        occupancy_tolerance: float = 0.0,
         site_tolerance: float = 1e-4,
         frac_tolerance: float = 1e-4,
         check_cif: bool = True,
@@ -296,7 +296,7 @@ def __init__(
         """
         Args:
             filename (str): CIF filename, gzipped or bzipped CIF files are fine too.
-            occupancy_tolerance (float): If total occupancy of a site is between 1 and occupancy_tolerance, the
+            occupancy_tolerance (float): If total occupancy of a site is between 1 and 1 + occupancy_tolerance, the
                 occupancies will be scaled down to 1.
             site_tolerance (float): This tolerance is used to determine if two sites are sitting in the same position,
                 in which case they will be combined to a single disordered site. Defaults to 1e-4.
@@ -1082,8 +1082,10 @@ def get_matching_coord(coord):
             all_species_noedit = all_species.copy()  # save copy before scaling in case of check_occu=False, used below
             for idx, species in enumerate(all_species):
                 total_occu = sum(species.values())
-                if 1 < total_occu <= self._occupancy_tolerance:
+                if 1 < total_occu <= 1 + self._occupancy_tolerance:
                     all_species[idx] = species / total_occu
+                elif total_occu > 1 + self._occupancy_tolerance:
+                    raise ValueError(f"Occupancy {total_occu} exceeded tolerance.")
 
         if all_species and len(all_species) == len(all_coords) and len(all_species) == len(all_magmoms):
             site_properties = {}

tests/io/test_cif.py

@@ -735,10 +735,10 @@ def test_bad_cif(self):
         filepath = f"{TEST_FILES_DIR}/cif/bad_occu.cif"
         parser = CifParser(filepath)
         with pytest.raises(
-            ValueError, match="No structure parsed for section 1 in CIF.\nSpecies occupancies sum to more than 1!"
+            ValueError, match="No structure parsed for section 1 in CIF.\nOccupancy 1.556 exceeded tolerance."
         ):
             parser.parse_structures(on_error="raise")
-        parser = CifParser(filepath, occupancy_tolerance=2)
+        parser = CifParser(filepath, occupancy_tolerance=1.0)
         struct = parser.parse_structures()[0]
         assert struct[0].species["Al3+"] == approx(0.778)
 
@@ -851,7 +851,7 @@ def test_no_check_occu(self):
             # should fail without setting custom occupancy tolerance
             CifParser.from_str(cif_str).parse_structures()
 
-        for tol in (1.5, 10):
+        for tol in (0.5, 9):
             parser = CifParser.from_str(cif_str, occupancy_tolerance=tol)
             structs = parser.parse_structures(primitive=False, check_occu=False)[0]
             assert structs[0].species.as_dict()["Te"] == 1.5