diff --git a/drivers/py/pes/harmonic.py b/drivers/py/pes/harmonic.py
index f200888d0..d92f7bac9 100644
--- a/drivers/py/pes/harmonic.py
+++ b/drivers/py/pes/harmonic.py
@@ -24,7 +24,7 @@ def check_arguments(self):
             self.k = float(self.args[0])
             self.type = "isotropic"
         elif len(self.args) == 3:
-            self.k = np.asarray(list(map(float,self.args)))
+            self.k = np.asarray(list(map(float, self.args)))
             self.type = "non-isotropic"
         else:
             sys.exit(self.error_msg)