Coupled cluster analysis of the U(1) lattice gauge model using a correlated "mean-field" reference state

We apply the coupled cluster method (CCM) to the U(1) lattice gauge model in 2+1 dimensions, basing our analysis on a reference state which already contains a certain class of single-plaquette correlations, the so-called {open_quote}{open_quote}mean-field{close_quote}{close_quote} reference state. We consider the simplest possible approximation scheme for the coupled cluster calculation, in which only single-plaquette correlations are included. The use of the {open_quote}{open_quote}mean-field{close_quote}{close_quote} reference state allows us to reproduce, within this level of approximation, the correct leading-order behavior of the ground-state energy of the equivalent Mathieu problem in {ital both} the strong- and weak-coupling limits. In addition, we have calculated the mean plaquette and the odd- and even-parity mass gaps. {copyright} {ital 1996 The American Physical Society.}