Sequential Cavity Method for Computing Free Energy and Surface Pressure

Abstract

We propose a new method for the problems of computing free energy and surface pressure for various statistical mechanics models on a lattice Zd [Z superscript d]. Our method is based on representing the free energy and surface pressure in terms of certain marginal probabilities in a suitably modi ed sublattice of Zd [Z superscript d]. Then recent deterministic algorithms for computing marginal probabilities are used to obtain numerical estimates of the quantities of interest. The method works under the assumption of Strong Spatial Mixing (SSP), which is a form of a correlation decay. We illustrate our method for the hard-core and monomer-dimer models, and improve several earlier estimates. For example we show that the exponent of the monomer-dimer coverings of Z3 [Z superscript 3] belongs to the interval [0:78595; 0:78599], improving best previously known estimate of (approx- imately) [0:7850; 0:7862] obtained in [FP05],[FKLM]. Moreover, we show that given a target additive error e [epsilon] > 0, the computational e ffort of our method for these two models is (1=e)O(1) [(1 / epsilon) superscript 0(1)] both for free energy and surface pressure. In contrast, prior methods, such as transfer matrix method, require exp ((1/e)0/1) [((1 / epsilon) superscript 0(1))] computation e ffort.