Equilibrium dynamics of the dissipative two-state system

Wilson's momentum shell renormalization group method is used to solve for the dynamics of the dissipative two--state system. We utilize the mapping of the spin--boson model onto the anisotropic Kondo model (AKM) and solve for the dynamics of the latter. We find that the AKM captures the physics of the dissipative two--state system for dissipation strength $0\leq\alpha\leq 4$ corresponding to $\infty\geq J_{\parallel}\geq -\infty$ in the AKM. The dynamics of the AKM shows a smooth crossover between two strong coupling regimes corresponding to weak and strong dissipation in the spin--boson model.