Thermochemical data in the required format must be provided for all chemical species included in a reaction mechanism. These entries have three purposes.

Thermochemical data can be provided either in the chemical kinetics input files (chem.inp or surf.inp), or in a more general thermodynamics data file, such as the therm.dat file provided in the data directory of the installed software. If thermodynamic data for a given chemical species is provided in both places, the data in the chemistry input file will override that in the general thermodynamic data file.

Every installation of an Ansys Chemkin application includes standard thermodynamic data files as starting points for simulations. These include:

The "standard" file, therm.dat, represents a historical collection of data accumulated by Sandia National Laboratories over the period from 1980 to 1995. The data fits in this collection of 1000+ species are based on a variety of sources, including JANAF Tables [11], NASA, and computational chemistry calculations performed at Sandia and elsewhere. Some species have duplicate entries with different names, reflecting alternate sets of thermochemical parameters. Except for a few labeling corrections, this file has deliberately not been updated to maintain backwards compatibility and avoid sudden alterations in users' results. Beginning with Ansys Chemkin 4, the therm.dat file now includes comments giving the CAS number (when available) for the chemical species. It also lists the standard heat of formation and entropy represented by the fits, as well as information on the source of the data.

The janaf_therm.dat file provides fits to thermodynamic data from the JANAF tables [11] for 890 gas-phase species. These are primarily small inorganic molecules, and ~640 are species that were not previously included in therm.dat. This file also includes comments giving the CAS number (when available) for the chemical species, as well as the standard heat of formation and entropy represented by the fits.

If a chemical species of interest is not included in one of the provided data files, further searching will be necessary. If the species of interest is included in a posted reaction mechanism on a web site, that data may be extracted from the original file and added to the user’s thermodata file of interest and used in other simulations. This approach is also likely to provide useful information on reaction kinetics of interest, but should be done with care. It is important to ensure that the chemical species in a reaction mechanism actually corresponds to the correct species identity intended. This can sometimes be difficult to determine in cases of larger molecules that have many possible isomers.

Beyond these Ansys Chemkin system files, there are a number of standard resources for thermochemical data. The Chemical Mechanism Data page lists some of the prominent online resources. One very useful source of thermochemical data is the web page of Professor Alexander Burcat of Technion Israeli Institute of Technology. Professor Burcat's database of thermodynamic properties is posted on an ftp site [12]. The data has been collected from many sources and is critically reviewed and frequently updated by Professor Burcat. The THERM.DAT file is in ready-to-use Chemkin format and contains the same data as in the BURCAT.THR file on this site. The BURCAT.THR file contain useful comments and descriptive text about each species, but that file would require some user manipulation (commenting out text lines) in order to use directly in Chemkin programs.

Another very useful online resource for thermodynamic data is the NIST Webbook [13]. This site provides thermochemical data in either SI or calorie-based units, so the units should be carefully noted and appropriately converted when data is downloaded and used. In addition to reviewing values for the enthalpy of formation and entropy at 298 K, the Webbook often provides heat capacity data, but using a different polynomial fitting equation, the Shomate equation. The FITDAT utility, described in Using Utilities of the Chemkin Input Manual, allows the conversion of such data into Ansys Chemkin format. The NASA Properties From Coefficients (PFC) program [12] is also a source of thermochemical data. Details of how to obtain thermochemical data from that site and convert them using the FITDAT utility are given in Using Utilities of the Chemkin Input Manual.

Useful printed compilations of thermochemical data for gas-phase and condensed species include the JANAF tables [11], books by Barin, et al. [15] and the CATCH [16] tables. In the original scientific literature, thermochemical data tend to be scattered across many decades of publications, and there is not always good agreement between various works. However, searching the scientific literature can lead to thermochemical data that are not in the standard compilations.

If the desired thermochemical data are not available in the literature for a species of interest, such data can be calculated or estimated using standard techniques. For smaller molecules, quantum chemistry calculations are a viable, though computationally intensive, approach to obtaining thermochemical data. The therm.dat file includes data for many species from such calculations, some of which are published in a series of papers from Sandia National Laboratories [17]. Thermochemical data from quantum chemistry calculations are also presented in a web site at the Combustion Research Facility at Sandia National Laboratories, [18] and a web site at NASA's Ames Research Center [19].

For other molecules, especially larger molecules composed of light elements, group additivity methods, as described by Benson [20], offer a computationally faster way of estimating thermochemical data. Group additivity methods are a systematic way of extrapolating properties of known species to chemically related species that have not been studied experimentally. For less-well studied chemical systems, the user can often use simple analogies with known species to estimate thermochemical parameters, but such values should be expected to have substantial uncertainties.

For any calculated or estimated thermochemistry, the FITDAT utility described in the next chapter can be used to produce the polynomial fits in the format required by the Ansys Chemkin software.

Thermochemical data for surface species are generally not found in standard references. As a result, thermochemical data for such species often are not included in the thermochemical database files (for example, therm.dat), but rather are included in the Surface Kinetics input file. In this way they are considered to be specific to that mechanism. Despite the dearth of thermochemical data, however, thermodynamic-data entries are required for any surface species, as the entries include information about species elemental composition in addition to their thermodynamic properties.

Users are generally required to generate their own thermochemistry for surface species. This can be done using statistical mechanics or group-additivity methods. In some cases, thermo data for surface species are available from a previously-developed reaction mechanism. However, the possibility of inconsistent reference states being used by different workers requires that extreme care be used in combining data from different mechanisms. There are also situations in which the surface reactions can be described using only irreversible reactions, and placeholder data used for thermochemical parameters. This can be done in cases where the simulation of interest does not involve solving the energy equation, or surface reactions are expected to have only minimal effects on the temperature.