def mmols(self):
        """ Dict of filepaths for all mmol files associated with code.

        Notes
        -----
        Downloads mmol files if not already present.

        Returns
        -------
        mmols_dict : dict, or None.
            Keys : int
                mmol number
            Values : str
                Filepath for the corresponding mmol file.
        """
        mmols_dict = {}
        mmol_dir = os.path.join(self.parent_dir, 'structures')
        if not os.path.exists(mmol_dir):
            os.makedirs(mmol_dir)
        mmol_file_names = ['{0}_{1}.mmol'.format(self.code, i) for i in range(1, self.number_of_mmols + 1)]
        mmol_files = [os.path.join(mmol_dir, x) for x in mmol_file_names]
        for i, mmol_file in enumerate(mmol_files):
            mmols_dict[i + 1] = mmol_file
            # If file does not exist yet, download the mmol and write to mmol_file.
            if not os.path.exists(mmol_file):
                get_mmol(self.code, mmol_number=i + 1, outfile=mmol_file)
        return mmols_dict