Optimal coupled-cluster approximation for the E⊗(b1+b2) Jahn-Teller effect

In this work we have applied the optimal coupled-cluster approximation to study the ground state of the E circle times (b(1) + b(2)) Jahn-Teller effect. The effectiveness of the optimal coupled-cluster approximation has been investigated for the whole range of the asymmetry parameter and various coupling strengths. It is shown that our results up to the third level of approximation are in very good agreement with the exact numerical diagonalization results and are better than those from earlier variational treatments. Furthermore, unlike previous variational treatments, the optimal coupled-cluster approximation has the advantage that the accurarcy of both the ground state energy and wavefunction estimates is being taken care of and can be systematically improved. Since the mathematical treatment in this work is simple, the optimal coupled-cluster approximation could be easily extended to the studies of other fermion-boson interacting systems, e.g the extended Jahn-Teller system.