Quantum vibrational chaos in the ArHCl van der Waals molecule

Quantum mechanical ro-vibrational calculations are presented for the ArHCl van der Waals complex using Hutson and Howard's empirical M5 potential. Analysis of the nodal structures, second differences and overlapping avoided crossings suggests that the higher bound states of ArHCl are chaotic. This chaos is made evident by perturbing the angular part of the kinetic energy term, e.g. by isotopic substitution or vibrational excitation of HCl. Calculations with J > 0 show increased level crossings but no significant increase in chaos because of an extra nearly good quantum number. The low-lying states of ArHCl appear to be poorly modelled by either the harmonic oscillator or free rotor approximations. Comparison with other molecules for which vibrational chaos has been predicted are made.