COUPLED-CLUSTER TREATMENT OF THE XY-MODEL

We study quantum spin systems in the 1D, 2D square and 3D cubic lattic es with nearest-neighbour XY-exchange. We use the coupled-cluster meth od (CCM) to calculate the ground-state energy, the T = 0 sublattice ma gnetization and the excited-state energies, all as functions of the an isotropy parameter gamma. We consider the case with S = 1/2 in detail and give some results for higher S. in 1D these results are compared w ith the exact S = 1/2 results and in 2D with Monte Carlo and series ex pansions. We obtain critical points close to the expected value gamma = 0 and our extrapolated LSUBn results for the ground-state energy are well converged for all gamma except very close to the critical point.