Computing Classical Partition Functions via Cycle-Space Sampling

We present a method for computing thermodynamic properties of classical spin systems by reformulating the partition function as a sum over even subgraphs of the interaction graph. Starting from the high-temperature expansion, we show that the expansion coefficients are determined by the weight distribution of the null space of the binary incidence matrix over F₂, and that this statement holds verbatim for multi-spin interactions once the graph is replaced by a hypergraph. We introduce a Monte Carlo scheme based on the $1/t$ Wang-Landau algorithm that performs a random walk directly in this null space. Because the weight distribution is temperature independent, a single estimate yields the zero-field partition function, and the thermodynamic quantities obtained from its temperature derivatives, at arbitrary temperature. We validate the approach against exact finite-size results on the two-dimensional triangular-lattice Ising model, apply it to the three- and four-dimensional hypercubic Ising models where no exact solution is available, and to the two- and three-dimensional Baxter-Wu models, whose three-spin interactions place them outside the reach of Pfaffian methods. For the two-body models we compare against conventional Wang-Landau sampling in energy space; for the Baxter-Wu models we validate against exact enumeration at the smallest size in each dimension.