The two principal functions of the Properties and Coefficients 1991 program (PAC91) are to provide a means of generating theoretical thermodynamic functions from molecular constant data and to supply a means of fitting these functions to empirical equations by using a least-squares fit. The coefficients obtained from the fit may then be used to generate a library of thermodynamic data in a uniform and easy-touse format for use in other computer codes. Several large compilations of selected or calculated thermodynamic data currently exist. Nevertheless, there is a continuing need for additional calculations due to the discovery of new species, the revision of existing molecular constant data and structural parameters, the need for data at temperatures other than those already published, the availability of new or revised heats of formation, dissociation or transition, and the revision of fundamental constants or atomic weights. Calculations may also be needed to compare the results of assuming various possible forms of the partition function. In addition, there is often a preference for thermodynamic data in functional rather than tabular form.

In order to satisfy these needs, the PAC91 program can perform any combination of the following:

1. calculate thermodynamic functions (heat capacity, enthalpy, entropy, and Gibbs energy) for any set of 1 to 202 temperatures,
2. obtain a least-squares fit of the first three of these functions (either individually, two at a time, or all three simultaneously) for up to eight temperature intervals, and
3. calculate, as a function of temperature, heats of formation and equilibrium constants from assigned reference elements.

The thermodynamic functions for ideal gases may be calculated from molecular constant data using one of several partition function variations provided by the program. For monatomic gases, one of three partition function cutoff techniques may be selected by the user, and unobserved but predicted electronic energy levels may be included by the program. For diatomic and polyatomic gases, one of five partition functions are available. These differ in the correction factors for nonrigid rotation, anharmonicity, and vibration-rotation interactions. Excited electronic states may also be included.

Several other capabilities of the program include the estimation of thermodynamic properties by a group additivity method, the ability to calculate properties for species with internal rotors, and a method for extrapolating data to high temperatures. For the purpose of additional processing, known thermodynamic functions for solids, liquids, or gases may be read in directly or thermodynamic functions may be calculated from heat capacity equations.