Date of Original Version
For Hamiltonian systems with two degrees of freedom, quantum invariants as constructed via time averages of dynamical variables in energy eigenstates provide a convenient representation of quantum nonintegrability effects in the form of a topographical map of quantized actions.
N. Srivastava and G. Müller. Quantum invariants: topographical map of quantized actions. Comments At. Mol. Phys. 25 (1990), 59-65.