Systematic polyad mixing in a local mode model

A system of two Morse oscillators with a potential coupling bilinear in the Morse variable yi (i=1,2) is studied in the space of creation and annihilation operators of the Morse functions. A rapidly converging expansion of yi in terms of these operators is obtained. This result allows us to establish a systematic approach to study polyad-breaking without including additional interactions. Possible applications of this approach are discussed.