diff --git a/psiflow/reference/reference.py b/psiflow/reference/reference.py
index d26fcbe..3220195 100644
--- a/psiflow/reference/reference.py
+++ b/psiflow/reference/reference.py
@@ -101,7 +101,7 @@ def compute(self, dataset: Dataset, *outputs: Optional[Union[str, tuple]]):
         if len(outputs_) == 1:
             return to_return[0]
         else:
-            return tuple(to_return)
+            return to_return
 
     def compute_atomic_energy(self, element, box_size=None):
         energies = []