On the static approximation for the Hubbard Hamiltonian

Abstract
We study the validity of the static approximation for the Hubbard model. We compare results obtained within the static approximation on two-dimensional finite lattices with results obtained from exact diagonalization and Monte Carlo simulations. We find that the static approximation is quite accurate at high temperatures but breaks down at low temperatures. One reason for its failure is that the static approximation destroys the rotational invariance in spin space.