On high temperature expansions for the Heisenberg model

A new method is developed for deriving high-temperature series expansions of the partition function for the Heisenberg model which avoids evaluating traces of products of non-commuting operators. The method is based on a rearrangement of the series as a sum of partition functions of finite clusters of steadily increasing order. The latter are then evaluated on a computer as traces of finite matrices. The calculations have been confined to spin ½. For loose-packed lattices, two new terms have been added to the zero-field series for the partition function and three new terms to the zero-field series for magnetic susceptibility.