docs: def to_pybel(self):
          """
          Produce a Pybel Molecule object.
  
          It is based on the capabilities of OpenBabel through Pybel. The present
          object must have at least `atomcoords`, `atomnos`, `charge` and `mult`
          defined.
  
          Returns
          -------
          `pybel.Molecule`
  
          Examples
          --------
          >>> from pyrrole.atoms import Atoms
          >>> dioxygen = Atoms({'atomcoords': [[0., 0., 0.],
          ...                                  [0., 0., 1.21]],
          ...                   'atomnos': [8, 8],
          ...                   'charge': 0,
          ...                   'mult': 3,
          ...                   'name': 'dioxygen'})
          >>> mol = dioxygen.to_pybel()
          >>> mol.molwt
          31.9988
  
          """
          # TODO: This only exports last geometry by default.
          obmol = _makeopenbabel(self.atomcoords, self.atomnos, self.charge,
                                 self.mult)
  
          title = self.name or ""
          if 'scfenergies' in self.attributes:
              title += ", scfenergy={} eV".format(self.scfenergies[-1])
          obmol.SetTitle(title)
  
          # TODO: make a test for this function.
          return _pb.Molecule(obmol)