diff --git a/src/matgl/ext/ase.py b/src/matgl/ext/ase.py
index 26d0a2a6..4d90fec6 100644
--- a/src/matgl/ext/ase.py
+++ b/src/matgl/ext/ase.py
@@ -178,7 +178,7 @@ def calculate(
             calc_result = self.potential(graph, lattice, state_attr_default)
         self.results.update(
             energy=calc_result[0].detach().cpu().numpy().item(),
-            free_energy=calc_result[0].detach().cpu().numpy(),
+            free_energy=calc_result[0].detach().cpu().numpy().item(),
             forces=calc_result[1].detach().cpu().numpy(),
         )
         if self.compute_stress: