Experimental and theoretical investigations of surface atom mean-square displacement (MSD) have been studied since the pioneer work of Kalashnikov many y e a r s ago /I/, using the Debye elastic continuum theory. Progress to understand better this problem was made afterwards using a lattice dynamical theory /2, 3/ and by numerical calculations based on a recursion method /4/. Further theoretical developments include an approach based on the Green functions technique 15, 6/. On the experimental side, much efforts have been made t o measure the Debye-Waller factor for several materials (for modern account of the experimental techniques see /7 t o S O / and the references therein).