Ion: bugfix for CO2(aq) reduced formula (materialsproject#4099)

Author: rkingsbury

src/pymatgen/core/ion.py

@@ -233,7 +233,7 @@ def get_reduced_formula_and_factor(
         elif formula == "HCOO":
             formula = "HCO2"
         # oxalate
-        elif formula == "CO2":
+        elif formula == "CO2" and self.charge == -2:
             formula = "C2O4"
             factor /= 2
         # diatomic gases

tests/core/test_ion.py

@@ -39,6 +39,8 @@ def test_charge_from_formula(self):
         assert Ion.from_formula("Ca++").charge == 2
         assert Ion.from_formula("Ca[++]").charge == 2
         assert Ion.from_formula("Ca2+").charge == 1
+        assert Ion.from_formula("C2O4-2").charge == -2
+        assert Ion.from_formula("CO2").charge == 0
 
         assert Ion.from_formula("Cl-").charge == -1
         assert Ion.from_formula("Cl[-]").charge == -1
@@ -70,7 +72,9 @@ def test_special_formulas(self):
             ("CH3COOH", "CH3COOH(aq)"),
             ("CH3OH", "CH3OH(aq)"),
             ("H4CO", "CH3OH(aq)"),
-            ("CO2-", "C2O4[-2]"),
+            ("C2O4--", "C2O4[-2]"),
+            ("CO2", "CO2(aq)"),
+            ("CO3--", "CO3[-2]"),
             ("CH4", "CH4(aq)"),
             ("NH4+", "NH4[+1]"),
             ("NH3", "NH3(aq)"),