The Coupled-Cluster Approximation: A Simple Approach to Spin-Lattice and Hubbard Models

Abstract

The coupled-cluster approximation (CCA) is applied to the ground state of strongly interacting fermions or spins on a regular lattice. A new scheme of successive CCAs, suitable for the lattice problem is proposed. Excellent approximations of the ground-state energies are obtained at the lowest levels in a straightforward way. Two examples are given: the antiferromagnetic Heisenberg linear chain and the half-filled Hubbard model on a two-dimensional square lattice, with comparison to exact results and/or other approximations.