ABSTRACT
This chapter develops partition functions for ensembles of single molecules and for chemical reactions between molecules. This greatly simplifies the ensemble method by avoiding enumeration of multiparticle microstates and their multiplicities, focusing directly on molecular configurations. The partition function associated with the single-molecule ensemble is often referred to as the "molecular partition function", and is represented by a lower case q. The chapter develops the so-called "molecular partition functions" for the canonical ensemble and the isothermal–isobaric ensemble, and shows how these simpler partition functions can be used to build multiparticle partition functions. The canonical and isothermal–isobaric partition functions allows to calculate thermodynamic properties of collections of N molecules by applying molecular models of the collection. The chapter shows how partition functions can be used to analyze systems in chemical equilibrium. It develops the quantitative relationship between molecular and multi-particle partition functions.
