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A New Principle to Determine the Variational Parameter for the Variational–Cumulant Expansion

The key point of the variational-cumulant expansion is the determination of the variational parameter. In this paper, we present the improved mean-field hypothesis (IMFH) which is the base to determine the parameter. The new method derived from the IMFH shows advantage over previous methods. The critical temperature and some thermal dynamical functions for the Heisenberg model are calculated with the new method.