2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/114534We prove that the electron density function of a real physical system can be uniquely determined by its values on any finite subsystem. This establishes the existence of a rigorous density-functional theory for any open electronic system. By introducing a new density functional for dissipative interactions between the reduced system and its environment, we subsequently develop a time-dependent density-functional theory which depends in principle only on the electron density of the reduced system. In the steady-state limit, the conventional first-principles nonequilibrium Green's function formulation for the current is recovered. A practical scheme is proposed for the new density functional: the wide-band limit approximation, which is applied to simulate the transient current through a model molecular device.10 pages, 6 figuresQuantum Physicstext