A systematic localized approximation scheme for the coupled-cluster treatment of quantum spin systems

Within the framework of the coupled-luster method a systematic localized approximation scheme, the so-called LSUBn scheme, is developed and applied to anisotropic quantum spin chains. The authors 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