diff --git a/cyten/models/degrees_of_freedom.py b/cyten/models/degrees_of_freedom.py
index 891e3ed4..00a3eaf6 100644
--- a/cyten/models/degrees_of_freedom.py
+++ b/cyten/models/degrees_of_freedom.py
@@ -29,6 +29,7 @@
     SymmetryError,
     U1Symmetry,
     ZNSymmetry,
+    Sector
 )
 from ..tensors import DiagonalTensor, SymmetricTensor
 from ..tools import as_immutable_array, is_iterable, to_iterable, to_valid_idx
@@ -149,6 +150,16 @@ def add_onsite_operator(
             )
         self.onsite_operators[name] = op
 
+    @abstractmethod
+    def get_mpo_building_block(self, label: str) -> SymmetricTensor:
+        """FIXME"""
+        ...
+
+    @abstractmethod
+    def mpo_building_block_basis(self, a: Sector, b: Sector) -> list[int]:
+        """"""
+        ...
+
     def state_index(self, label: str | int) -> int:
         """The index of a basis state."""
         if isinstance(label, str):