diff --git a/psiflow/functions.py b/psiflow/functions.py
index 06d0975..71a74fd 100644
--- a/psiflow/functions.py
+++ b/psiflow/functions.py
@@ -122,7 +122,7 @@ def geometry_to_key(geometry: Geometry) -> tuple:
 
             plumed_ = self.plumed_instances[key]
             if geometry.periodic:
-                cell = np.copy(geometry.cell).astype(np.float64)
+                cell = np.copy(geometry.cell).astype(np.float64).copy()
                 plumed_.cmd("setBox", cell)
 
             # set positions