  We prove the existence of the exact density-functional theory formalism for
open electronic systems, and develop subsequently an exact time-dependent
density-functional theory (TDDFT) formulation for the dynamic response. The
TDDFT formulation depends in principle only on the electron density of the
reduced system. Based on the nonequilibrium Green's function technique, it is
expressed in the form of the equation of motion for the reduced single-electron
density matrix, and this provides thus an efficient numerical approach to
calculate the dynamic properties of open electronic systems. In the
steady-state limit, the conventional first-principles nonequilibrium Green's
function formulation for the current is recovered.
