GRADIENT EXPANSION FOR T-S[N] - CONVERGENCE STUDY FOR JELLIUM SPHERES

The convergence of the gradient expansion (GE) for the kinetic energy functional T(s)[n] is tested on the basis of the spherical jellium mod el for metal clusters. By insertion of Kohn-Sham densities into the GE it is found that fourth-order contributions to the GE are more import ant for jellium spheres than for atoms, indicating that these correcti ons might also be relevant for the description of solids. By solution of the Euler-Lagrange equations resulting from the GE truncated at sec ond or fourth order, it is demonstrated that the variational accuracy of the GE is considerably lower than that obtained by insertion of hig h-quality densities. Furthermore, it is shown that a GE to second orde r with an adjusted prefactor of the gradient term does not give variat ional results for jellium s heres superior to a fourth-order GE as in the case of atoms.