“…If one is interested in obtaining the partition sum, without obtaining spin configurations, one can also treat systems with full periodic boundary conditions in polynomial time, by using transfer-matrix approaches 24,25,26,27 , but the running time is again strongly increasing, limiting the investigations to small systems. The most recent studies are based on matching algorithms 29,30,31,32,33,34,35,36,37 , while other exact approaches can be found in Refs. 38,39,40 .…”