COUPLED-CLUSTER METHOD FOR QUANTUM LATTICES - APPLICATION TO SQUARE S=1/2 HEISENBERG ANTIFERROMAGNETS

A coupled-cluster formalism similar to that used in electronic-structure theory is developed for quantum-mechanical systems localized to sites on a lattice. It is shown how diagrammatic methods may be used to represent the cluster contributions in a convenient and systematic manner. The formalism is applied to the square S = 1/2 Heisenberg antiferromagnet, yielding with minor computational effort results of quality comparable to those from computationally intensive quantum Monte Carlo calculations.