Methods for calculating nonconcave entropies

Abstract

Five different methods which can be used to analytically calculate entropies that are nonconcave as functions of the energy in the thermodynamic limit are discussed and compared. The five methods are based on the following ideas and techniques: i) microcanonical contraction, ii) metastable branches of the free energy, iii) generalized canonical ensembles with specific illustrations involving the so-called Gaussian and Betrag ensembles, iv) restricted canonical ensemble, and v) inverse Laplace transform. A simple long-range spin model having a nonconcave entropy is used to illustrate each method.

v1: 22 pages, IOP style, 7 color figures, contribution for the JSTAT special issue on Long-range interacting systems. v2: Open problem and references added, minor typos corrected, close to published version

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