Comparison of ab-Initio and Phenomenological Calculations of Surface Phonons

In this paper we will address to the problem of the physical meaning of the force constants by comparing the forces acting betwen atoms both in the surface region and in the bulk, calculated in the framework of a sophisticated force constant model and the corresponding forces evaluated in the framework of an a-priori pseudopotential theory, where the electron-ion system is treated perturbatively by considering the effect of the electronic screening and the roel od the electron-electron term on the electron-phono interaction.