Approximate method of free energy calculation for spin system with   arbitrary connection matrix
  The proposed method of the free energy calculation is based on the approximation of the energy distribution in the microcanonical ensemble by the Gaussian distribution. We hope that our approach will be effective for the systems with long-range interaction, where large coordination number q ensures the correctness of the central limit theorem application. However, the method provides good results also for systems with short-range interaction when the number q is not so large. 
Boris Kryzhanovsky and Leonid Litinskii
cond-mat.dis-nn