Reduced coupling constant , with decoupling

These nonresonant couplings may be dealt with by a Van Vleck (see Section 4.2) or contact transformation (Nielsen, 1951), which folds their effects into systematically quasi-degenerate groups of states called polyads (see Section 9.4.5). The polyad Heff fit model accounts accurately for the observed energy levels (and many other properties) of an entire family of scaling-related polyads. In effect, the dimensionality of the exact H is drastically reduced in the polyad Heff. This reduction is due to the existence of several approximate constants of motion which permit H to be block diagonalized into families of dynamically decoupled polyad Heff matrices. 

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