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"

def set_auxiliary_basis_set(self, folder, auxiliary_folder,
                                auxiliary_basis_set_type="aug_cc_pvtz"):
        """
        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"
        """

        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")