diff --git a/esm/utils/structure/molecular_complex.py b/esm/utils/structure/molecular_complex.py
index dc679d83..16c8d73d 100644
--- a/esm/utils/structure/molecular_complex.py
+++ b/esm/utils/structure/molecular_complex.py
@@ -870,6 +870,10 @@ def to_mmcif(self) -> str:
         atom_array.atom_name = np.array(atom_names, dtype="U4")
         atom_array.add_annotation("b_factor", dtype=float)
         atom_array.b_factor = atom_bfactors
+        atom_array.add_annotation("occupancy", dtype=float)
+        atom_array.occupancy = np.ones(
+            n_atoms, dtype=np.float32
+        )  # Necessary for BioPython MMCIFParser
         atom_array.add_annotation("entity_id", dtype=int)
         atom_array.entity_id = atom_entity_ids
 
diff --git a/esm/utils/structure/protein_chain.py b/esm/utils/structure/protein_chain.py
index 08f39df2..a4872d93 100644
--- a/esm/utils/structure/protein_chain.py
+++ b/esm/utils/structure/protein_chain.py
@@ -227,6 +227,7 @@ def atom_array(self) -> bs.AtomArray:
                     atom_name=residue_constants.atom_types[i],
                     element=residue_constants.atom_types[i][0],
                     b_factor=float(b_factor),
+                    occupancy=1.0,  # Necessary for BioPython MMCIFParser
                 )
                 atoms.append(atom)
         return bs.array(atoms)
@@ -262,6 +263,7 @@ def atom_array_no_insertions(self) -> bs.AtomArray:
                     atom_name=residue_constants.atom_types[i],
                     element=residue_constants.atom_types[i][0],
                     b_factor=float(b_factor),
+                    occupancy=1.0,  # Necessary for BioPython MMCIFParser
                 )
                 atoms.append(atom)
         return bs.array(atoms)