How to use the parmed.openmm.OpenMMParameterSet.from_parameterset function in ParmEd

def generate_gaff_xml():
    """
    Return a file-like object for `gaff.xml`
    """
    #TODO this function isn't used anywhere
    from openmoltools import amber
    gaff_dat_filename = amber.find_gaff_dat()

    # Generate ffxml file contents for parmchk-generated frcmod output.
    leaprc = StringIO("parm = loadamberparams %s" % gaff_dat_filename)
    import parmed
    params = parmed.amber.AmberParameterSet.from_leaprc(leaprc)
    params = parmed.openmm.OpenMMParameterSet.from_parameterset(params)
    citations = """\
    Wang, J., Wang, W., Kollman P. A.; Case, D. A. "Automatic atom type and bond type perception in molecular mechanical calculations". Journal of Molecular Graphics and Modelling , 25, 2006, 247260.
    Wang, J., Wolf, R. M.; Caldwell, J. W.;Kollman, P. A.; Case, D. A. "Development and testing of a general AMBER force field". Journal of Computational Chemistry, 25, 2004, 1157-1174.
    """
    ffxml = str()
    gaff_xml = StringIO(ffxml)
    provenance=dict(OriginalFile='gaff.dat', Reference=citations)
    params.write(gaff_xml, provenance=provenance)

    return gaff_xml

charge = atom.GetPartialCharge()
        parameters = { 'charge' : charge }
        atom_template = ForceField._TemplateAtomData(atomname, typename, element, parameters)
        template.atoms.append(atom_template)
    for bond in molecule.GetBonds():
        if (bond.GetBgn() in residue_atoms) and (bond.GetEnd() in residue_atoms):
            template.addBondByName(bond.GetBgn().GetName(), bond.GetEnd().GetName())
        elif (bond.GetBgn() in residue_atoms) and (bond.GetEnd() not in residue_atoms):
            template.addExternalBondByName(bond.GetBgn().GetName())
        elif (bond.GetBgn() not in residue_atoms) and (bond.GetEnd() in residue_atoms):
            template.addExternalBondByName(bond.GetEnd().GetName())

    # Generate ffxml file contents for parmchk-generated frcmod output.
    leaprc = StringIO('parm = loadamberparams %s' % frcmod_filename)
    params = parmed.amber.AmberParameterSet.from_leaprc(leaprc)
    params = parmed.openmm.OpenMMParameterSet.from_parameterset(params, remediate_residues=False)
    ffxml = StringIO()
    params.write(ffxml)
    return template, ffxml.getvalue()

frcmod_filename     = prefix + '.frcmod'

        # Write Tripos mol2 file as antechamber input.
        _writeMolecule(molecule, input_mol2_filename, standardize=normalize)

        # Parameterize the molecule with antechamber.
        run_antechamber(prefix, input_mol2_filename, charge_method=None, net_charge=net_charge, gaff_mol2_filename=gaff_mol2_filename, frcmod_filename=frcmod_filename, gaff_version=gaff_version, resname=True)

        # Append to leaprc input for parmed.
        leaprc += '%s = loadmol2 %s\n' % (prefix, gaff_mol2_filename)
        leaprc += 'loadamberparams %s\n' % frcmod_filename

    # Generate ffxml file contents for parmchk-generated frcmod output.
    leaprc = StringIO(leaprc)
    params = parmed.amber.AmberParameterSet.from_leaprc(leaprc)
    params = parmed.openmm.OpenMMParameterSet.from_parameterset(params, remediate_residues=False)

    ffxml = StringIO()
    params.write(ffxml)

    # TODO: Clean up temporary directory.
    os.chdir(olddir)

    if ignoreFailures:
        return ffxml.getvalue(), failed_molecule_list
    else:
        return ffxml.getvalue()