A SYSTEMATIC LOCALIZED APPROXIMATION SCHEME FOR THE COUPLED-CLUSTER TREATMENT OF QUANTUM SPIN SYSTEMS

Within the framework of the coupled-cluster method a systematic localized approximation scheme, the so-called LSUBn scheme, is developed and applied to anisotropic quantum spin chains. We use computer generation of the terms in the coupled non-linear equations to obtain successive approximations up to order n = 8. The results for the ground-state energy show systematic and rapid convergence in the Ising-like region. The method also gives good results in the planar region where perturbation theory is not valid, and only breaks down for values of the anisotropy parameter-DELTA less than or similar -0.5. We show that for a given value of n the LSUBn scheme reproduces exactly the corresponding nth order of large-DELTA perturbation theory around the Ising limit.