Assign frame properties to molecule/structure when indexing trajectory (

#3979)

* fea: assign frame properties to molecule/structure when indexing trajectory.

* test: test that the frame properties are carried over when indexing a single frame

src/pymatgen/core/trajectory.py

@@ -219,6 +219,7 @@ def __getitem__(self, frames: ValidIndex) -> Molecule | Structure | Self:
                     charge=charge,
                     spin_multiplicity=spin,
                     site_properties=self._get_site_props(frames),  # type: ignore[arg-type]
+                    properties=None if self.frame_properties is None else self.frame_properties[frames],
                 )
 
             lattice = self.lattice if self.constant_lattice else self.lattice[frames]
@@ -228,6 +229,7 @@ def __getitem__(self, frames: ValidIndex) -> Molecule | Structure | Self:
                 self.species,
                 self.coords[frames],
                 site_properties=self._get_site_props(frames),  # type: ignore[arg-type]
+                properties=None if self.frame_properties is None else self.frame_properties[frames],
                 to_unit_cell=True,
             )
 

tests/core/test_trajectory.py

@@ -213,6 +213,10 @@ def test_frame_properties(self):
         expected = props[1:]
         assert traj[1:].frame_properties == expected
 
+        # test that the frame properties are set correctly when indexing an individual structure/molecule
+        expected = props[0]
+        assert traj[0].properties == expected
+
     def test_extend(self):
         traj = copy.deepcopy(self.traj)