Memory formulas for perturbations in time-dependent density functional theory

Exact time-dependent density functionals depend on both the entire history of the density and the initial wave function. Recently we have shown that the two effects are entangled by an exact condition that should be useful as a test in building accurate memory-dependent functionals. Here we consider this condition for small perturbations and derive an exact relation connecting the exchange-correlation kernel with initial-state derivatives. We discuss other exact conditions on the initial-state derivatives which shed some light on the elusive memory-dependence in time-dependent density functional theory. (c) 2005 Wiley Periodicals, Inc.