Exchange–correlation corrections to lattice dynamics of simple metals, and a search for soft modes at normal and expanded volume

Using a density-based local pseudopotential, we study the effect of local field or exchange–correlation corrections on lattice dynamics for several simple metals (the alkalis, aluminum, and lead). Turning on exchange and correlation in the dielectric function significantly reduces phonon frequencies for some metals. We consider three levels of description: the local density approximation (LDA), the generalized gradient approximation (GGA), and the nearly exact. Especially for the lower-density metals, the LDA linear response introduces negligible error into the phonon frequencies when exchange and correlation are treated together. This fact results from an almost complete cancellation between the nonlocalities of exchange and correlation. We find no static charge density wave in any of these metals at normal volume, even in the most favorable exchange-only case. For Na and Al at a volume about 50% bigger than normal, we find that a transverse phonon goes soft on the [110] zone boundary. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 359–369, 1998