diff --git a/TB2J/io_merge.py b/TB2J/io_merge.py
index f5e5e9b..6de6256 100644
--- a/TB2J/io_merge.py
+++ b/TB2J/io_merge.py
@@ -4,6 +4,7 @@
 from itertools import product
 from TB2J.io_exchange import SpinIO
 
+I = np.eye(3)
 uz = np.array([[0.0, 0.0, 1.0]])
 
 def get_rotation_matrix(magmoms):