Fourth-order gradient expansion of the fermion kinetic energy: Extra terms for nonanalytic densities

The fourth-order density-gradient expansion of the fermion kinetic energy contains terms which integrate over space to zero for analytic densities, but not for the densities of atoms or of those metal-surface models for which the expansion has been tested. Evaluation of these terms shows that the fourth-order expansion is not quite as accurate as it has been believed to be.