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.

def localopt(self, forcefield='mmff94', steps=500):
        """
        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.
        """
        pbmol = pb.Molecule(self._obmol)
        pbmol.localopt(forcefield=forcefield, steps=steps)
        self._obmol = pbmol.OBMol