In addition to specification of the thermodynamic variables (e.g. (T,P), (T,V)) and initial feed concentrations, an equilibrium calculation requires free-energy data for the set of species considered. This data may be provided in several ways:

• values of the standard free energy of formation of each species
• values of the so-called free-energy function for each species
• values of equilibrium constants for a set of specified reactions

Sources for free-energy data are discussed in detail in several places, including Chapter 3 of Chemical Reaction Equilibrium Analysis; Theory and Algorithms, W. R. Smith and R. W. Missen, Krieger, 1991; a reprint, with corrections, of the same title by Wiley-Interscience, 1982. EQS4WIN contains a built-in database that calculates the first type of data listed above, which includes the approximately 1800 species listed in the JANAF tables (JANAF Thermochemical Tables, 3rd. ed., M. W. Chase et. al., J. Phys. Chem. Ref. Data, Volume 14, 1985, Supplement No. 1, Am. Chem. Society, Washington, D.C.). In fact, the EQS4WIN database is more accurate than this version of the tables, since it contains corrections to errors for 17 species that were discovered by Mathtrek Systems during the preparation of the EQS4WIN database.

For many types of problems, in particular those arising in analytical chemistry, aquatic chemistry, biochemistry, and geochemistry, often only data of the last type listed above is available. Although this type of data can be converted into an internally consistent set of data of the first type, as discussed in Section 9.4 of the Smith and Missen reference quoted above, there remains a tendency in the literature of the fields listed to regard equilibrium problems that supply this type of data as requiring different solution methods than those appropriate to the other two. For example, in the recent text, Thermodynamics in Geochemistry, G. M. Anderson and D. A. Crerar, Oxford University Press, 1993, on p. 517 it is stated "There is an important distinction between free energy based programs and those using equilibrium constants, and this often determines which method to use in specific applications. The free energy programs actually require much more fundamental thermodynamic information, and this can restrict their usefulness." Similar remarks appear in Section 13.3 of Geochemical Thermodynamics, 2nd ed., D. K. Nordstrom and J. L. Munoz, Blackwell Scientific Publications, Oxford, 1994. Such statements illustrate the narrow view that still exists in several fields regarding equilibrium calculations.

By implementing the research findings of Smith and Missen concerning these problems, EQS4WIN incorporates an algorithm that allows the input of any of the 3 types of free-energy data listed above, bringing all these problem types under a single theoretical viewpoint. Illustrations of the use of reaction input data are contained in the Analytical Chemistry, Aquatic chemistry and Biochemistry examples.