MULTISITE CORRELATION-FUNCTIONS IN 2-DIMENSIONAL LATTICE GASES

The three- and four-site-correlation functions in a two-dimensional Is ing-Peierls square lattice gas are computed by means of the cluster va riation method (CVM) introduced by Kikuchi, The non-linear CVM equatio ns are numerically solved and the values of several multi-site correla tion functions are compared with the results of Monte Carlo simulation s at four different coverages (theta = 1/8, 1/4, 1/2, 3/4) as function s of the lateral coupling beta J. The results obtained by CVM are accu rate within few percent at all coverages and lateral couplings taken i nto account, Thus the CVM results are used as a basis for a discussion of the validity of the Kirkwood superposition approximation in lattic e gases.