def node_desc(self, atoms):
        """default 9 bits descriptor
        7 bits of atomic number (0-127) and 2 bits of pi electrons (0-3)
        """
        a1 = self.mol.atom(atoms[0])
        a2 = self.mol.atom(atoms[1])
        a1t = a1.number << 2 | a1.pi
        a2t = a2.number << 2 | a2.pi
        pair = sorted((a1t, a2t))
        return pair[0] << 9 | pair[1]