Dynamical motion of atoms in surfaces: A model of W (100) as an example

For a model of the (100) surface of tungsten, we have studied the mean-square displacement of atoms and its anisotropy as a function of distance from the surface. In addition, we study correlations in the motion of nearest and next-nearest neighbors in and near the surface. These calculations employ the continued-fraction method to construct the relevant spectral densities. We assess the accuracy and efficiency of the method, and analysis of the structures in the various spectral-density functions provides insight into the modes which control displacement amplitudes and correlations in the surface.