High-order coupled-cluster method for general spin-lattice problems: An illustration via the anisotropic Heisenberg model

A high-order coupled cluster method (CCM) for the zero-temperature ground states of quantum lattice spin systems of general spin quantum number (s greater than or equal to 1/2) is described and illustrated via the spin-1 anisotropic Heisenberg antiferromagnet on the square lattice, Results for the ground-state energy and sublattice magnetization are seen to be in excellent agreement with the best of other approximate methods. Furthermore, we are able to follow the solution to the CCM equations at a given level of approximation with respect to the anisotropy parameter Delta from the trivial Ising limit (Delta --> infinity) down to a critical value Delta (c), at which point the physical solution terminates. The results for the ground-state energy, sublattice magnetization, and phase transition point thus support our choice of this model as a good nontrivial test case of our CCM formalism.