TIME-DEPENDENT KOHN-SHAM FORMALISM

Within density-functional theory a time-dependent perturbation theory is derived that yields exact orbital-dependent expressions for the tim e-dependent exchange-correlation potential in the form of a perturbati on series. This leads to an exact time-dependent Kohn-Sham scheme base d on perturbation theory. In first order an ''exchange-only'' Kohn-Sha m procedure is obtained, that is a generalization of a recently presen ted time-dependent optimized potential method. The exact, in higher or der, but quite complicated expressions for the exchange-correlation po tential may serve as a starting point for the construction of approxim ate exchange-correlation potentials. An adiabatic connection or coupli ng constant path between a time-dependent physical system and its corr esponding Kohn-Sham system is introduced.