Slippage of initial conditions for the Redfield master equation

For a slow open quantum subsystem weakly coupled to a fast thermal bath, we derive the general form of the slippage to be applied to the initial condi tions of the Redfield master equation. This slippage is given by a superope rator which describes the non-Markovian dynamics of the subsystem during th e short-time relaxation of the thermal bath. We verify in an example that t he Redfield equation preserves positivity after the slippage superoperator has been applied to the initial density matrix of the subsystem. For delta- correlated baths, the Redfield master equation reduces to the Lindblad mast er equation and the slippage of initial conditions vanishes consistently. ( C) 1999 American Institute of Physics. [S0021-9606(99)50137-2].