code: def mmols(self):
          
          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
docs: 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.