Positive and negative parity states of the Two-Rotor Model and scissors   modes
  In previous investigations of the Two-Rotor Model with axially symmetric rotors the wave functions were assumed to be invariant under inversion of the axes of the rotors, which restricted the spectrum to positive parity states. We relax this condition requiring only that the wave functions be invariant under the square of the inversion operators. As a result we get positive as well as negative parity states that are all split by tunneling effect. 
Fabrizio Palumbo
nucl-th