  A general concept for the derivation of symmetry-based pseudo spin
Hamiltonians is described. It systematically bridges the gap between the
atomistic basis and various pseudo spin models presented in literature. It thus
allows the application of the multitude of analytical and numerical procedures
derived for the statistical mechanics treatment of the latter to the
description of structurally modulated crystals and furnishes a general frame
for reviewing these models from a common point of view.
