Nonlocal kinetic energy functional for nonhomogeneous electron systems

We present three density-functional models for the kinetic energy of the electron gas. The density functionals are constructed to give the exact energy and linear response of homogeneous systems. We include nonlocal behavior through an ``averaged density'' evaluated with the aid of a weight function. By a scaling hypothesis we obtain a universal (i.e., density independent) differential equation for the weight functions in k space. We have used these three functionals for jellium surfaces and light atoms to compare the results with the exact results and the calculations of other functional approximations.