diff --git a/cp2analisi.py b/cp2analisi.py
index d4bc330a7..7717f62d7 100644
--- a/cp2analisi.py
+++ b/cp2analisi.py
@@ -135,5 +135,5 @@ def file_length( filename ):
                                     data['cell'],
                                     False,
                                     False)
-analisi_traj.write_lammps_binary(sys.argv[1]+'.bin', 0,-1)
+analisi_traj.write_lammps_binary(sys.argv[2]+'.bin', 0,-1)