ONE-ELECTRON EXCITATIONS, CORRELATION-EFFECTS, AND THE PLASMON IN CESIUM METAL

We study the dynamical electronic response of Cs at a first-principles level. The spatially localized 5p semicore shell induces a physical i nterplay between crystal local fields and electron correlations, leadi ng to a novel, and relatively large, many-body shift of the plasmon en ergy. This effect, combined with that of one-electron transitions into empty states near the plasmon energy, yields a plasmon dispersion cur ve which is in excellent agreement with experiment for small wave vect ors. In addition, our results feature a flat dispersion for large wave vectors, in qualitative agreement with experiment.