The exact expression of kinetic energy as a functional of the density for a system of noninteracting fermions, which has been searched for many years, is derived based on the direct method.
Explicit kinetic-energy density functionals for noninteracting electron systems that are based on reproducing the linear response function in the homogeneous limit are the most accurate approximations to the exact Kohn-Sham functional. In this work we study several such models for one-dimensional systems. In this case, the geometrical simplicity allows the calculation of the static linear response resulting from these functionals for inhomogeneous systems. Likewise, the discussion of the energies and density profiles obtained is easier and clearer than that for more realistic problems. Special attention is paid to the application of explicit kinetic energy functionals to systems with odd and noninteger number of particles.
It is known in density-functional theory that the noninteracting kinetic-energy density functional ${T}_{s}[\ensuremath{\rho}]$ is not first-degree homogeneous in density scaling. However, it is shown here that, for every particle number N, there is an N-particle noninteracting kinetic-energy density functional ${T}_{N}[\ensuremath{\rho}],$ that is, a density functional that gives the noninteracting kinetic energy for N-particle densities, which is of first-degree homogeneity in the density $\ensuremath{\rho}(\stackrel{\ensuremath{\rightharpoonup}}{r}).$ This gives a powerful tool, a strong requirement, for constructing such functionals. A systematic procedure to obtain the real part of ${T}_{N}[\ensuremath{\rho}],$ the full ${T}_{N}[\ensuremath{\rho}]$ in one-dimension, for each N is also proposed. It is pointed out, further, that in the Euler-Lagrange equations that determine the one-particle orbitals that define ${T}_{s}[\ensuremath{\rho}],$ the Lagrange multiplier that forces the orbitals to yield $\ensuremath{\rho}(\stackrel{\ensuremath{\rightharpoonup}}{r})$ is not other than the first derivative of ${T}_{s}[\ensuremath{\rho}],$ $\ensuremath{\delta}{T}_{s}[\ensuremath{\rho}]/\ensuremath{\delta}\ensuremath{\rho}(\stackrel{\ensuremath{\rightharpoonup}}{r}),$ which yields a natural derivation of the Kohn-Sham equations. Utilizing the same idea, it is shown for ground states how the Schr\"odinger equation can be derived from the basics of density-functional theory as well.