code: def node_desc(self, atoms):
          
          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]
docs: default 9 bits descriptor
          7 bits of atomic number (0-127) and 2 bits of pi electrons (0-3)