diff --git a/src/tad_dftd3/reference.py b/src/tad_dftd3/reference.py
index 5c2d67a..298e922 100644
--- a/src/tad_dftd3/reference.py
+++ b/src/tad_dftd3/reference.py
@@ -20,6 +20,7 @@
 C6 dispersion coefficients.
 """
 import os.path as op
+
 import torch
 
 from ._typing import Any, NoReturn, Optional, Tensor