Quantum-Classical Transition in Dissipative Double-Well Systems --A   Numerical Study by a New Monte Carlo Scheme--
  Thermodynamics of dissipative quantum systems with double-well potentials is studied by the path-integral Monte Carlo (PIMC) method without truncation to the two-state model. For efficient simulation at low temperatures, we develop a new scheme of local update based on approximate decomposition of the Boltzmann weight to the Gaussian distributions. Localization transition induced by ohmic dissipation is clarified numerically for arbitrary potential barriers. 
Takeshi Matsuo, Yuhei Natsume and Takeo Kato