diff --git a/pysisyphus/diabatization/coulomb_eta.py b/pysisyphus/diabatization/coulomb_eta.py
index 49c658251..543ad97c2 100755
--- a/pysisyphus/diabatization/coulomb_eta.py
+++ b/pysisyphus/diabatization/coulomb_eta.py
@@ -26,6 +26,10 @@
 from pysisyphus.wavefunction.localization import JacobiSweepResult
 
 
+# TODO: rename eta to epsilon. Why did I call all this ER-eta instead of
+# ER-epsilon?
+
+
 # Wrap gradient and Hessian functions, so they only have to be set up
 # once and can be reused later.
 __jac_func = None
diff --git a/pysisyphus/io/cube.py b/pysisyphus/io/cube.py
index 6d04e6d65..7c1c0d425 100644
--- a/pysisyphus/io/cube.py
+++ b/pysisyphus/io/cube.py
@@ -165,6 +165,10 @@ class Cube:
     comment1: str = "Generated by pysisyphus"
     comment2: str = ""
 
+    @property
+    def vol_element(self):
+        return np.prod(np.diag(self.axes))
+
     @staticmethod
     def from_file(fn):
         return parse_cube(fn)
diff --git a/tests/test_diabatization/test_diabatization.py b/tests/test_diabatization/test_diabatization.py
index 63c7d11a3..5c65ea8b0 100644
--- a/tests/test_diabatization/test_diabatization.py
+++ b/tests/test_diabatization/test_diabatization.py
@@ -51,6 +51,7 @@ def test_diabatization_driver(dia_kinds, this_dir):
         cube_kinds=dia_driver.CubeKind.NONE,
         out_dir=out_dir,
         force=True,
+        h5_fn=out_dir / "00_bena2_cat_dia_result.h5",
     )
     # Only one key will be present
     key = list(dia_results.keys())[0]