Lower-bound approximation to the free energy of the three-dimensional Ising model

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Abstract
We use the one-hypercube variational approximation to the renormalization-group equations to calculate a lower bound for the zero-field free energy of an Ising model on a bcc lattice. Our results for the free energy, energy and specific heat are in good agreement with those obtained by series expansions. Estimates are given for the critical exponents for the situation in which the variational parameter is varied in linearizing around the fixed point.