DERIVING THE ISOTHERMAL-ISOBARIC ENSEMBLE - THE REQUIREMENT OF A SHELL MOLECULE AND APPLICABILITY TO SMALL SYSTEMS

The isothermal-isobaric ensemble is derived from the canonical ensembl e instead of the customary microcanonical ensemble. By explicitly acco unting for interactions between the system and the external bath, an i sothermal-isobaric partition function is obtained that is more general than a previous derivation and is better suited to study small system s. The ensemble requires a ''wall'' or ''shell'' molecule to identify and count nonredundantly all the volumes and associated configurations of the system, verifying the volume scale obtained by Koper and Reiss [J. Phys. Chem. 100, 422 (1996)] (under the condition of minimal inte raction between the bath and system for which it is valid). In additio n, the failure of information theory to arrive at the proper volume sc ale (or ''shell'' molecule) for the ensemble (again when interaction b etween the bath and system is ignored) is demonstrated. (C) 1998 Ameri can Institute of Physics.