VOLUME FORCES IN SIMPLE METALS

Within the local pseudopotential model taken to second order, the metallic energy, U, is a sum of volume dependent and a structure dependent term. The contribution from the volume dependence of U to the longitudinal elastic constants C//1//1 and c//1//2 and the bulk modulus B is evaluated for a variety of models. This contribution, DELTA //b//s, is due to the volume dependence of the screening. The usual dynamical matrix is generalized to incorporate this volume dependence. The volume effects are important for long wave phonons only and for most practical purposes may be neglected otherwise. The volume contributions to the pressure and the deviation of the cauchy relation are also evaluated suggesting that the observed deviation of the Cauchy relation does not provide a reliable estimate of DELTA //b//s.