Representation and Poly-time Approximation for Pressure of \(\mathbb {Z}^2\) Lattice Models in the Non-uniqueness Region

We develop a new pressure representation theorem for nearest-neighbour Gibbs interactions and apply this to obtain the existence of efficient algorithms for approximating the pressure in the 2-dimensional ferromagnetic Potts, multi-type Widom–Rowlinson and hard-core models. For Potts model, our results apply to every inverse temperature but the critical. For Widom–Rowlinson and hard-core models, they apply to certain subsets of both the subcritical and supercritical regions. The main novelty of our work is in the latter.