Inapproximability After Uniqueness Phase Transition in Two-Spin Systems

A two-state spin system is specified by a 2 x 2 matrix A = {A{0,0} A{0,1}, A{1,0} A{1,1}} = {\beta 1, 1 \gamma} where \beta, \gamma \ge 0. Given an input graph G=(V,E), the partition function ZA(G) of a system is defined as ZA(G) = \sum{\sigma: V -> {0,1}} \prod{(u,v) \in E} A{\sigma(u), \sigma(v)} We prove inapproximability results for the partition function in the region specified by the non-uniqueness condition from phase transition for the Gibbs measure. More specifically, assuming NP \ne RP, for any fixed \beta, \gamma in the unit square, there is no randomized polynomial-time algorithm that approximates ZA(G) for d-regular graphs G with relative error \epsilon = 10^{-4}, if d = \Omega(\Delta(\beta,\gamma)), where \Delta(\beta,\gamma) > 1/(1-\beta\gamma) is the uniqueness threshold. Up to a constant factor, this hardness result confirms the conjecture that the uniqueness phase transition coincides with the transition from computational tractability to intractability for Z_A(G). We also show a matching inapproximability result for a region of parameters \beta, \gamma outside the unit square, and all our results generalize to partition functions with an external field.