Correctly compute rotational symmetry number for D*h point group (#4093)

* Correctly compute rotational symmetry number for D*h point group

Correct value is 2

* pre-commit auto-fixes

* Add docs

* 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/symmetry/analyzer.py

@@ -1329,6 +1329,15 @@ def get_symmetry_operations(self) -> Sequence[SymmOp]:
         return generate_full_symmops(self.symmops, self.tol)
 
     def get_rotational_symmetry_number(self) -> int:
+        """Get rotational symmetry number.
+
+        Returns:
+            int: Rotational symmetry number.
+        """
+        if self.sch_symbol == "D*h":
+            # Special case. H2 for example has rotational symmetry number 2
+            return 2
+
         """Get the rotational symmetry number."""
         symm_ops = self.get_symmetry_operations()
         symm_number = 0

tests/symmetry/test_analyzer.py

@@ -530,6 +530,7 @@ def test_tricky(self):
         mol = Molecule.from_file(f"{TEST_DIR}/dh.xyz")
         pg_analyzer = PointGroupAnalyzer(mol, 0.1)
         assert pg_analyzer.sch_symbol == "D*h"
+        assert pg_analyzer.get_rotational_symmetry_number() == 2
 
     def test_linear(self):
         coords = [
@@ -540,9 +541,12 @@ def test_linear(self):
         mol = Molecule(["C", "H", "H"], coords)
         pg_analyzer = PointGroupAnalyzer(mol)
         assert pg_analyzer.sch_symbol == "D*h"
+        assert pg_analyzer.get_rotational_symmetry_number() == 2
+
         mol = Molecule(["C", "H", "N"], coords)
         pg_analyzer = PointGroupAnalyzer(mol)
         assert pg_analyzer.sch_symbol == "C*v"
+        assert pg_analyzer.get_rotational_symmetry_number() == 1
 
     def test_asym_top(self):
         coords = [