diff --git a/src/somd2/config/_config.py b/src/somd2/config/_config.py
index 96b410e..181a051 100644
--- a/src/somd2/config/_config.py
+++ b/src/somd2/config/_config.py
@@ -1257,9 +1257,6 @@ def pert_file(self, pert_file):
 
         self._pert_file = pert_file
 
-        if pert_file is not None:
-            self._somd1_compatibility = True
-
     @property
     def save_energy_components(self):
         return self._save_energy_components