def term_symbols(self):
        """
        All possible  Russell-Saunders term symbol of the Element
        eg. L = 1, n_e = 2 (s2)
        returns
           [['1D2'], ['3P0', '3P1', '3P2'], ['1S0']]

        """
        L_symbols = 'SPDFGHIKLMNOQRTUVWXYZ'

        L, v_e = self.valence

        # for one electron in subshell L
        ml = list(range(-L, L + 1))
        ms = [1 / 2, -1 / 2]
        # all possible configurations of ml,ms for one e in subshell L
        ml_ms = list(product(ml, ms))

        # Number of possible configurations for r electrons in subshell L.
        n = (2 * L + 1) * 2
        # the combination of n_e electrons configurations
        # C^{n}_{n_e}
        e_config_combs = list(combinations(range(n), v_e))

        # Total ML = sum(ml1, ml2), Total MS = sum(ms1, ms2)
        TL = [sum([ml_ms[comb[e]][0] for e in range(v_e)])
              for comb in e_config_combs]
        TS = [sum([ml_ms[comb[e]][1] for e in range(v_e)])
              for comb in e_config_combs]
        comb_counter = Counter([r for r in zip(TL, TS)])

        term_symbols = []
        while sum(comb_counter.values()) > 0:
            # Start from the lowest freq combination,
            # which corresponds to largest abs(L) and smallest abs(S)
            L, S = min(comb_counter)

            J = list(np.arange(abs(L - S), abs(L) + abs(S) + 1))
            term_symbols.append([str(int(2 * (abs(S)) + 1))
                                 + L_symbols[abs(L)]
                                 + str(j) for j in J])
            # Without J
            # term_symbols.append(str(int(2 * (abs(S)) + 1)) \
            #                     + L_symbols[abs(L)])

            # Delete all configurations included in this term
            for ML in range(-L, L - 1, -1):
                for MS in np.arange(S, -S + 1, 1):
                    if (ML, MS) in comb_counter:

                        comb_counter[(ML, MS)] -= 1
                        if comb_counter[(ML, MS)] == 0:
                            del comb_counter[(ML, MS)]
        return term_symbols
All possible  Russell-Saunders term symbol of the Element
        eg. L = 1, n_e = 2 (s2)
        returns
           [['1D2'], ['3P0', '3P1', '3P2'], ['1S0']]