COMPARISON BETWEEN PHENOMENOLOGICAL AND PSEUDOPOTENTIAL FORCE-CONSTANTS FOR THE LATTICE-DYNAMICS OF AL

In this paper we present a critical comparison of,calculations of phon on-dispersion curves for aluminum, based on pseudopotential and empiri cal force-constant methods. The former method is.based on both perturb ative and nonperturbative evaluations of the total energy of the cryst al. In the perturbative approach the total energy is evaluated to seco nd order in the electron-ion interaction with a local pseudopotential. In the nonperturbative approach the electron-ion interaction is treat ed exactly using a non-local ab initio pseudopotential. In the empiric al force-constant method the total energy is represented by a sum of t wo-body and three-body terms, the latter being restricted to interacti ons among triplets of nearest neighbors and of nearest and next-neares t neighbors. All models include physical mechanisms for ensuring the b reakdown of the Cauchy relation. We show that in each method convergen ce of the calculated phonon frequencies requires the inclusion of long -range interactions. The radial-force constants obtained by the empiri cal and perturbative pseudopotential methods are very similar for the first three shells of neighbors. On the other hand, the tangential for ce constants differ markedly, a reflection of the different physical m echanisms that come into play in the various models in the establishme nt of equilibrium.