Fix chemical system method for different oxidation states (#3915)

* Fix chemical system when the same element appears in multiple oxidation states

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Matthew Horton <mkhorton@users.noreply.github.com>

src/pymatgen/core/composition.py

@@ -487,7 +487,7 @@ def chemical_system(self) -> str:
         sorted alphabetically and joined by dashes, by convention for use
         in database keys.
         """
-        return "-".join(sorted(el.symbol for el in self.elements))
+        return "-".join(sorted(self.chemical_system_set))
 
     @property
     def num_atoms(self) -> float:

tests/core/test_composition.py

@@ -701,6 +701,7 @@ def test_elements(self):
     def test_chemical_system(self):
         assert Composition({"Na": 1, "Cl": 1}).chemical_system == "Cl-Na"
         assert Composition({"Na+": 1, "Cl-": 1}).chemical_system == "Cl-Na"
+        assert Composition({"Na+": 1, "Na2+": 1, "Cl-": 1}).chemical_system == "Cl-Na"
 
     def test_chemical_system_set(self):
         assert Composition({"Na": 1, "Cl": 1}).chemical_system_set == {"Cl", "Na"}