  The exact form of the kinetic energy functional has remained elusive in
orbital-free models of density functional theory (DFT). This has been the main
stumbling block for the development of a general-purpose framework on this
basis. Here, we show that on the basis of a two-density model, which represents
many-electron systems by mass density and spin density components, we can
derive the exact form of such a functional. The exact functional is shown to
contain previously suggested functionals to some extent, with the notable
exception of the Thomas-Fermi kinetic energy functional.
