We study the perturbative corrections to the weak coupling limit type
Gorini-Kossakowski-Sudarshan-Lindblad equation for the reduced density matrix
of an open system. For the spin-boson model in the rotating wave approximation
at zero temperature we show that the perturbative part of the density matrix
satisfies the time-independent Gorini-Kossakowski-Sudarshan-Lindblad equation
for arbitrary order of the perturbation theory if all the moments of the
reservoir correlation function are finite. But the initial condition for
perturbative part of the density matrix does not only differ from that for the
whole density matrix, but also fails to be a density matrix under certain
resonance conditions