Date of Original Version
The energy‐level sequence, whose spacings distribution is the most frequently invoked indicator of quantum chaos, can be derived (for systems with two degrees of freedom) from a two‐dimensional representation of quantum invariants by projection. In this representation, such properties of level sequences as effective randomness in integrable models and level repulsion in nonintegrable models can be more directly interpreted in terms of physical properties. In integrable models, anharmonicities convert quasiperiodic level sequences into effectively random sequences.
Niraj Srivastava, Charles Kaufman and Gerhard Müller. Chaos in spin clusters: quantum invariants and level statistics. J. Appl. Phys. 67 (1990), 5627-5629.
Available at: http://dx.doi.org/10.1063/1.345907