COMPARISON BETWEEN LINKED-CLUSTER EXPANSION METHODS FOR THE HEISENBERG-ANTIFERROMAGNET ON THE SQUARE LATTICE

Comparisons are made between the results of three different linked-clu ster expansion methods, as applied to the S = 1/2 Heisenberg antiferro magnet on a square lattice at zero temperature. Connected moments of t he Hamiltonian are calculated up to 15th order, extending previous res ults of Lee and Lo, and are used to form a t expansion. The t expansio n is found to be less accurate than the coupled cluster expansion, whi ch in turn is less accurate than the standard perturbation expansion f or this system.