Large-Ucluster-Hamiltonian expansion of the Hubbard model

We discuss an expansion of the low-energy effective Hamiltonian of the large-U Hubbard model, in which higher-order terms involve lattice clusters with larger numbers of sites rather than larger powers of the hopping amplitude parameter t. Expressions for each term, valid to all orders in t, can be obtained by exactly diagonalizing the Hubbard model for small clusters. Leading terms in this expansion are derived. \textcopyright{} 1996 The American Physical Society.