Density functional theory and its extension in the nonequilibrium regime, time-dependent density functional theory, are powerful tools for predicting the structures, energies and dynamics of electronic systems. Their usefulness derives from the Kohn-Sham scheme whereby a system of real, interacting particles is replaced by a fictitious system of non-interacting particles subject to an effective external potential instead of a pairwise particle-particle interaction. The Kohn-Sham universe yields the same observable phenomena as that predicted by standard quantum mechanics so long as the effective external potential is known. However, for the vast majority of systems it is not known, and the usually local (in time and space) functional approximations employed do not capture the physics of true nonlocal interactions.
In this thesis, the exact charge and current densities of model quantum transport devices described by nonlocal potentials are studied and methods for reverse-engineering the corresponding exact Kohn-Sham effective external potential for time-dependent and steady-state density functional theory approaches to the same systems are presented, as well as the resulting exact potentials themselves. Features of improved functionals for calculating approximate Kohn-Sham systems are demonstrated. These functionals are suggested to be very different from existing functionals employed, describing not potentials but electric and magnetic fields, and have a strong dependence on the local and semilocal charge and current density
