diff --git a/mmtbx/geometry_restraints/mopac_manager.py b/mmtbx/geometry_restraints/mopac_manager.py
index a8ae5615c0..5a03e5fc87 100644
--- a/mmtbx/geometry_restraints/mopac_manager.py
+++ b/mmtbx/geometry_restraints/mopac_manager.py
@@ -52,7 +52,7 @@ def _input_header(self, gradients_only=False):
                         'quintet', # - 4 unpaired electrons
                         'sextet', # - 5 unpaired electrons
                         'septet',
-                        ][self.multiplicity]
+                        ][self.multiplicity] + ' UHF'
     additional_options=''
     if gradients_only:
       additional_options+=' 1SCF GRAD ANALYT'