diff --git a/CHANGELOG.md b/CHANGELOG.md
index 477b4ea..a387199 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Changed
 
-- Add `rcut` option to `build_random_cell`.
+- Add mandatory `rcut` option to `build_random_cell`.
 
 
 ## [0.1.0] - 2024-10-06
diff --git a/src/tinyff/atomsmithy.py b/src/tinyff/atomsmithy.py
index d1d8b1b..df91c27 100644
--- a/src/tinyff/atomsmithy.py
+++ b/src/tinyff/atomsmithy.py
@@ -97,8 +97,8 @@ def __call__(self, dist: ArrayLike) -> tuple[NDArray, NDArray]:
 def build_random_cell(
     cell_length: float,
     natom: int,
+    rcut: float,
     *,
-    rcut: float | None = None,
     maxiter: int = 100,
     rng: np.random.Generator | None = None,
 ):
@@ -109,8 +109,6 @@ def build_random_cell(
     atpos0 = rng.uniform(0, cell_length, (natom, 3))
 
     # Define cost function to push the atoms appart.
-    if rcut is None:
-        rcut = 0.49 * cell_length
     pwff = PairwiseForceField(PushPotential(rcut), rcut)
 
     def costgrad(atpos_raveled):
diff --git a/tests/test_atomsmithy.py b/tests/test_atomsmithy.py
index a89f25a..a2f51b0 100644
--- a/tests/test_atomsmithy.py
+++ b/tests/test_atomsmithy.py
@@ -128,6 +128,6 @@ def test_push_cutoff():
 
 def test_random_box():
     rng = np.random.default_rng(42)
-    atpos = build_random_cell(10.0, 32, rng=rng)
+    atpos = build_random_cell(10.0, 32, 3.0, rng=rng)
     nlist = build_nlist_simple(atpos, [10.0, 10.0, 10.0], rcut=4.0)
     assert nlist["dist"].min() > 2.0