Optimal coupled-cluster approximation for the ground-state properties of aspin-boson model

In the present work we have developed an optimal coupled-cluster approximat ion, which can take care of both the accuracies of the ground-state energy and the wavefunction estimates, for the ground state of a two-state system coupled to a dispersionless boson bath. This new approach is also able to g ive a tight upper bound to the ground-state energy of the system. Up to the fourth level of this approximation our results show excellent agreement wi th the numerical exact diagonalization results. In particular, our results suggest no discontinuous localization-delocalization transition of the two- state system. This is consistent with the exact result.