  A novel approximation scheme is proposed to describe the dynamics of the
spin-boson problem. Being nonperturbative in the coupling strength nor in the
tunneling frequency, it gives reliable results over a wide regime of
temperatures and coupling strength to the thermal environment for a large class
of bath spectral densities.
  We use a path-integral approach and start from the exact solution for the
two-level system population difference in the form of a generalized master
equation (GME). Then, we approximate inter-blip and blip-sojourns interactions
up to linear order, while retaining all intra-blip correlations to find the
kernels entering the GME in analytical form. Our approximation scheme, which we
call Weakly-Interacting Blip Approximation (WIBA), fully agrees with
conventional perturbative approximations in the tunneling matrix element
(Non-Interacting Blip Approximation) or in the system-bath coupling strength.
