Physical-Cluster Theory for a Reactive Gas

Abstract

Physical‐cluster expansions are given for the thermodynamic properties and equation of state of a reactive fluid. This provides a statistical‐mechanical characterization of the fluid in terms of the stable atomic aggregates or molecules which actually are present rather than in terms of the more familiar, but less intuitive, Ursell—Mayer cluster integrals. Here, the Ursell—Mayer cluster development is applied only to the molecular interactions and not to the stronger ``chemical'' forces which are responsible for the stability of these aggregates. Detailed attention is given to a system composed exclusively of monomers and dimers. Formulas are derived for all of the coefficients needed to determine the first‐order density corrections to the equation of state and composition of the fluid. Numerical results are reported for the special case of a square‐well atom—atom interaction and a nonadditive three‐body potential which is rigid and repulsive. Finally, we present a more general, quantum‐mechanical version of the theory.