Cumulant Expansions and the Spin-Boson Problem
  The dynamics of the dissipative two-level system at zero temperature is studied using three different cumulant expansion techniques. The relative merits and drawbacks of each technique are discussed. It is found that a new technique, the non-crossing cumulant expansion, appears to embody the virtues of the more standard cumulant methods. 
D.R. Reichman, F.L.H. Brown, and P. Neu