Fundamental treatment of the isoenthalpic-isobaric ensemble

Fluctuation formulas which are characteristic of the classical isoenthalphic-isobaric ensemble are derived in a direct and fundamental way. The technique used is adapted from a new method introduced by Pearson, Halicioglu, and Tiller [Phys. Rev. A 32, 3030 (1985)] in their treatment of the microcanonical ensemble. The application of this new method to other adiabatic ensembles is also discussed.