The Thermodynamic Property Estimator utility in Ansys Chemkin relies on several estimation techniques.
An estimation of boiling point and critical properties based on Group Additivity.
The second is the estimation of the Lennard-Jones diameter and well depth, based on the critical properties.
The definition of the acentric factor when the Vapor Pressure expression is given explicitly.
The estimated acentric factors from a Lee Kesler vapor pressure relation.
The estimation of Boiling Point, Freezing Point, Critical Temperature, Critical Pressure, and Critical Volume are all taken from the Joback group contributions [7]. These are described in Equation 3–31 through Equation 3–35 , below
 | (3–31) |
 | (3–32) |
 | (3–33) |
 | (3–34) |
 | (3–35) |
Here, each 
parameter represents the corresponding parameter associated with a
functional group, 
is boiling point, 
is freezing point, 
is critical temperature, 
is
critical pressure, and 
is critical volume.
Lennard Jones properties are estimated from the critical property correlations outlined by Chung et al. [21]
 | (3–36) |
 | (3–37) |
The 
is the Lennard Jones well depth, and 
is the Lennard Jones radius.
Joback group contributions report average errors of 12.9 K for boiling point, 4.8 K for critical temperature, 2.1 bar for critical pressure, and 7.5 cm3/mole for critical volume. The Chung correlations report an average error in derived viscosities of 1.9%.
The acentric factor definition is given in Equation 3–38 :
 | (3–38) |
Where 
is the reduced temperature 
. This expression can be used if the vapor pressure correlation is explicitly
stated in the thermo section of the chemistry mechanism. This definition is the default method
of estimating the acentric factor.
When the vapor pressure is not known, a Lee-Kesler relation for vapor pressure can be used to estimate the acentric factor.
 | (3–39) |
 | (3–40) |
 | (3–41) |
Where 
is 
.
Cetane number estimation is done in a similar manner as critical properties, in that the Cetane number is estimated based on chemical structure. The estimation assumes a linear correlation between the chemical structure and the ignition predicted by the cetane number.
This fit was re-created during release 2024 R2 when a third party chem-informatics package was replaced, and again in 2025 R1 to reduce error, so older versions may not produce identical results. The current data fitting was performed on IQT data from a NIST compendium [22] of cetane numbers and produced a root mean squared error of less than 5.