code: def set_auxiliary_basis_set(self, folder, auxiliary_folder,
                                  auxiliary_basis_set_type="aug_cc_pvtz"):
          
  
          list_files = os.listdir(auxiliary_folder)
  
          for specie in self._mol.symbol_set:
              for file in list_files:
                  if file.upper().find(
                                  specie.upper() + "2") != -1 and file.lower().find(
                          auxiliary_basis_set_type) != -1:
                      shutil.copyfile(auxiliary_folder + "/" + file,
                                      folder + "/" + specie + "2.ion")
docs: copy in the desired folder the needed auxiliary basis set "X2.ion" where X is a specie.
          :param auxiliary_folder: folder where the auxiliary basis sets are stored
          :param auxiliary_basis_set_type: type of basis set (string to be found in the extension of the file name; must be in lower case)
                 ex: C2.ion_aug_cc_pvtz_RI_Weigend find "aug_cc_pvtz"