def dump_system(self, filepath=None, modular=False, **kwargs):
        """
        Dump a :class:`MolecularSystem` to a file (PDB or XYZ).

        Kwargs are passed to :func:`pywindow.io_tools.Output.dump2file()`.

        Parameters
        ----------
        filepath : :class:`str`
           The filepath for the dumped file. If :class:`None`, the file is
           dumped localy with :attr:`system_id` as filename.
           (defualt=None)

        modular : :class:`bool`
            If False, dump the :class:`MolecularSystem` as in
            :attr:`MolecularSystem.system`, if True, dump the
            :class:`MolecularSystem` as catenated :class:Molecule objects
            from :attr:`MolecularSystem.molecules`

        Returns
        -------
        None : :class:`NoneType`

        """
        # If no filepath is provided we create one.
        if filepath is None:
            filepath = '/'.join((os.getcwd(), str(self.system_id)))
            filepath = '.'.join((filepath, 'pdb'))
        # If modular is True substitute the molecular data for modular one.
        system_dict = deepcopy(self.system)
        if modular is True:
            elements = np.array([])
            atom_ids = np.array([])
            coor = np.array([]).reshape(0, 3)
            for mol_ in self.molecules:
                mol = self.molecules[mol_]
                elements = np.concatenate((elements, mol.mol['elements']))
                atom_ids = np.concatenate((atom_ids, mol.mol['atom_ids']))
                coor = np.concatenate((coor, mol.mol['coordinates']), axis=0)
            system_dict['elements'] = elements
            system_dict['atom_ids'] = atom_ids
            system_dict['coordinates'] = coor
        # Check if there is an 'atom_ids' keyword in the self.mol dict.
        # Otherwise pass to the dump2file atom_ids='elements'.
        # This is mostly for XYZ files and not deciphered trajectories.
        if 'atom_ids' not in system_dict.keys():
            atom_ids = 'elements'
        else:
            atom_ids = 'atom_ids'
        # Dump system into a file.
        self._Output.dump2file(
            system_dict, filepath, atom_ids=atom_ids, **kwargs)