code: def localopt(self, forcefield='mmff94', steps=500):
          
          pbmol = pb.Molecule(self._obmol)
          pbmol.localopt(forcefield=forcefield, steps=steps)
          self._obmol = pbmol.OBMol
docs: A wrapper to pybel's localopt method to optimize a Molecule.
  
          Args:
              forcefield: Default is mmff94. Options are 'gaff', 'ghemical',
                  'mmff94', 'mmff94s', and 'uff'.
              steps: Default is 500.