diff --git a/mess/primitive.py b/mess/primitive.py
index 1338091..86a7d39 100644
--- a/mess/primitive.py
+++ b/mess/primitive.py
@@ -16,6 +16,7 @@ class Primitive(eqx.Module):
     alpha: float = eqx.field(converter=asfparray, default=1.0)
     lmn: Int3 = eqx.field(converter=asintarray, default=(0, 0, 0))
     norm: Optional[float] = None
+    atom_index: Optional[int] = None
 
     def __post_init__(self):
         if self.norm is None: