Ground state of a dissipative two-level system: Coupled-cluster approximation

The ground state of a two-level system coupled to a dispersionless phonon bath is studied by the coupled-cluster method. The estimates of both the ground-state energy and the tunneling reduction factor are found to be in good agreement with the exact values. It is also found that within the coupled-cluster approximation there is no indication of the discontinuous localization-delocalization transition. This is consistent with the exact result. © 1996 The American Physical Society.