The Complexity of Ferromagnetic Ising with Local Fields

We consider the complexity of approximating the partition function of the ferromagnetic Ising model with varying interaction energies and local external magnetic fields. Jerrum and Sinclair provided a fully polynomial randomised approximation scheme for the case in which the system is consistent in the sense that the local external fields all favour the same spin. We characterise the complexity of the general problem by showing that it is equivalent in complexity to the problem of approximately counting independent sets in bipartite graphs, thus it is complete in a logically-defined subclass of #P previously studied by Dyer, Goldberg, Greenhill and Jerrum. By contrast, we show that the corresponding computational task for the q-state Potts model with local external magnetic fields and q > 2 is complete for all of #P with respect to approximation-preserving reductions.