"The phonons are well known as the quanta of lattice vibrations. These lattice vibrations are the most understood from successful theories of condensed matter physics and can be considered as a milestone in the comprehension of the properties of condensed materials. Ranging from the infrared, Raman, neutron and in the recent years, synchrotron spectra to the anharmonic properties as well as the electron phonon interactions and superconductivity, there is little left in which the phonons do not play a vital role. In order to understand the material properties, however, the modelling of microscopic interactions intrinsically involved between electrons and ions, remained an ambitious and affordable tool over decades. One of the remarkable developments in physics of phonons is the concept of Rigid Shell Model (RSM) ... derived from the original crystal lattice theory of Born and Huang ..., which includes the microscopic electron-phonon interactions in several classes of non-metallic solids."