Statistical Theory of the Atom in Momentum Space

We investigate a semiclassical momentum density energy functional for atoms and show that it yields the same value as the well-known Thomas-Fermi functional. In fact, we show an explicit relation between the minimizers of the two functionals. The main result is that the atomic momentum density converges on the scale $Z^{2\mathrm{/}3}$ to the minimizer of the momentum density energy functional. This allows to determine the linear response of atoms on momentum dependent perturbations.