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physica status solidi (b)

A Variational Linear Approach for the Free Energy Applied to the Ising Model

J. R. Faleiro Ferreira

Published Aug 1, 1988• 6 citations
Publisher

Abstract

Abstract A recently proposed variational approach is applied to the Ising model. Former results may be improved when a sum of linear chain Hamiltonians is used as an auxiliary Hamiltonian instead of a sum of single particle Hamiltonians.
DOI
10.1002/pssb.2221480230

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To calculate thermodynamic functions in statistical mechanics the authors propose a new variational approach which has as particular cases the approaches of Bogoliubov, Oguchi and Tsallis-da Silva (1982), but introduces some improvements, mainly in the short-range order effects above the critical temperature. It requires a choice of a trial Hamiltonian that defines an approximate ensemble through which the thermodynamic functions may be calculated. To check the efficiency of their approach they apply it to the ferromagnetic Ising model by using the simplest possible trial Hamiltonian.
In the context of the Hamiltonian variational approach we propose a systematic method for the improvement of any given trial wave function. The approach has many similarities with the Lanczos scheme and it may be used for quantum-mechanical problems as well as field-theory and statistical-mechanics systems. We apply the method to the Mathieu equation and to the Ising one-dimensional model in a finite lattice. The agreement between our results and the exact ones is excellent in the whole range of parameters. We also briefly discuss the application of these ideas to lattice gauge theories and the similarities with other recently proposed methods.
The modified Kadanoff's variational method recently considered by us is applied to calculate the free energies and their derivatives for a three-dimensional Ising model with bcc structure and without external magnetic field. We find a critical point at kc=0.162134{k}_{c}=0.162134kc​=0.162134 and the associated critical indices $\ensuremath{\alpha}=0.12$, ${\ensuremath{\alpha}}^{\ensuremath{'}}=0.095$.