On the use of the quasi-Gaussian entropy theory in noncanonical ensembles. I. Prediction of temperature dependence of thermodynamic properties

In previous articles we derived and tested the quasi-Gaussian entropy theory, a description of the excess Helmholtz free energy in terms of the potential energy distribution, instead of the configurational partition function. We obtained in this way the temperature dependence of thermodynamic functions in the canonical ensemble assuming a Gaussian, Gamma or Inverse Gaussian distribution. In this article we extend the theory to describe the temperature dependence of thermodynamic properties in an exact way in the isothermal-isobaric and grand canonical ensemble, using the distribution of the appropriate heat function. For both ensembles restrictions on and implications of these distributions are discussed, and the thermodynamics assuming a Gaussian or (diverging) Gamma distribution is derived.: These cases have been tested for water at constant pressure, and the results for the latter case are satisfactory. Also the distribution of the heat function of some theoretical model systems is considered. (C) 1998 American Institute of Physics. [S0021-9606(98)50232-2].