Allow more than 20 terminal groups in CompoundPhaseDiagram (#4081)

* Allow more than 20 terminal groups in CompoundPhaseDiagram

* Allow more than 20 terminal groups in CompoundPhaseDiagram

* pre-commit auto-fixes

---------

Co-authored-by: Sasha Fonari <fonari@schrodinger.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/pymatgen/analysis/phase_diagram.py

@@ -1471,6 +1471,34 @@ def __init__(self, entries, terminal_compositions, normalize_terminal_compositio
         self.species_mapping = species_mapping
         super().__init__(p_entries, elements=species_mapping.values())
 
+    @staticmethod
+    def num2str(num):
+        """
+        Convert number to a list of letter(s). First letter must be `f`.
+
+        :param num int: Number to convert
+
+        :rtype: str
+        :return Converted string
+        """
+
+        # `f` letter was hard coded, do not modify. Alphabet consists of letters
+        # between f and z.
+        letter = "f"
+        code, z_code = ord(letter), ord("z") + 1
+
+        rest_num_letters = z_code - code
+        mult = num // rest_num_letters
+
+        ret = letter * mult if mult else ""
+
+        remainder = num % rest_num_letters + 1
+
+        start = code - 1
+        ret += chr(start + remainder)
+
+        return ret
+
     def transform_entries(self, entries, terminal_compositions):
         """
         Method to transform all entries to the composition coordinate in the
@@ -1493,7 +1521,7 @@ def transform_entries(self, entries, terminal_compositions):
         # Map terminal compositions to unique dummy species.
         sp_mapping = {}
         for idx, comp in enumerate(terminal_compositions):
-            sp_mapping[comp] = DummySpecies("X" + chr(102 + idx))
+            sp_mapping[comp] = DummySpecies("X" + self.num2str(idx))
 
         for entry in entries:
             if getattr(entry, "attribute", None) is None:

tests/analysis/test_phase_diagram.py

@@ -701,6 +701,18 @@ def test_get_formation_energy(self):
     def test_str(self):
         assert str(self.pd) == "Xf-Xg phase diagram\n4 stable phases: \nLiFeO2, Li2O, Li5FeO4, Fe2O3"
 
+    def test_num2str(self):
+        ret = set()
+
+        num = 100
+        for idx in range(num):
+            val = CompoundPhaseDiagram.num2str(idx)
+            assert val.isalpha()
+            assert ord(val[0]) >= ord("f")
+            ret.add(val)
+
+        assert len(ret) == num
+
 
 class TestPatchedPhaseDiagram(TestCase):
     def setUp(self):