Helicity decoupling approximation

It is possible to formulate the bound state problem for a wide range of Van der Waals complexes in a form reminiscent of that for atom-diatom systems. The similarities between the different sets of coupled equations will be helpful in understanding the dynamical approximations that may be applied to the larger complexes. In particular, the helicity decoupling approximation, which has proved to be accurate for most atom-diatom systems, is equally applicable to the larger systems. 

The theory behind body-fixed representations and the associated angular momentum function expansions of the wavefunction (or wave packet) in terms of bases parameterized by the relevant constants of the motion and approximate constants of the motion is highly technical. Some pertinent results will simply be stated. The two good constants of the motion are total angular momentum, J, and parity, p = +1 or 1. An approximate constant of the motion is K, the body-fixed projection of total angular momentum on the body-fixed axis. For simplicity, we will restrict attention to the helicity-decoupled or centrifugal sudden (CS) approximation in which K can be assumed to be a constant of the motion. In terms of aU its components, and the iteration number k, the real wave packet is taken to be [21]... 

Several decoupling approximations have been developed to simplify treatments where many rotational channels are coupled to begin with. A number of calculations have used orbital-rotational decoupling in the body-fixed frame, an il-dominant decoupling, and helicity decoupling. Several of these approaches have been recast in terms of effective Hamiltonians. Other decoupling treatments have extended the distorted-wave approximation by means of exponential operators or with optical potentials. 

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