The DSUBm approximation scheme for the coupled cluster method and applications to quantum magnets

A new approximate scheme, DSUBm, is described for the coupled cluster method. We apply it to two well- studied (spin-1/2 Heisenberg antiferromagnet) spin-lattice models, namely: the XXZ and the XY models on the square lattice in two dimensions. Results are obtained in each case for the ground-state energy, the sublattice magnetization and the quantum critical point. They are in good agreement with those from such al- ternative methods as spin-wave theory, series expansions, exact diagonalization techniques, quantum Monte Carlo methods and those from the CCM using the LSUBm scheme.