  Starting from the basic Landau model for the incommensurate-commensurate
materials of the class II, we derive the spectrum of collective modes for all
(meta)stable states from the corresponding phase diagram. It is shown that all
incommensurate states posses Goldstone modes with acoustic dispersions. The
representation in terms of collective modes is also used in the calculation and
discussion of static dielectric response for systems with the commensurate wave
number in the center of the Brillouin zone.
