Answers are grounded in this paper and cite their sources.
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.
No takes yet. Share an insight, caveat, or question.
Perdew et al. (1986) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: