There are some good rules of thumb that explain the subroutine naming conventions. All subroutine names (with the exception of PKINDX) begin with the letters CK so that Gas-phase Kinetics subroutines are easily recognized and so that they are likely different from any user subroutine names. The four remaining letters identify the purpose of the subroutine: The first one or two usually refer to the variable that is being computed; the last letters refer to either the input variables or the units.

State variables are denoted by P (pressure), T (temperature), Y ( mass fraction), X ( mole fraction), and C ( molar concentration). Thermodynamic properties are referred to by CP and CV (Specific heats), H (enthalpy), S (entropy), U (internal energy), G (Gibbs free energy), and A (Helmholtz free energy). The thermodynamic property subroutines may be called to return properties in mass units, denoted by MS or S as the last letter(s), or in molar units, denoted by ML or L as the last letter(s). The letter B (for the bar as in ) in a thermodynamic property subroutine name indicates that it returns mean mixture properties.

Subroutines that return net chemical production rates have a W (for ) following the CK, and routines that return creation and destruction rates or creation rates and destruction times have a CD or a CT, respectively, following the CK. Rate-of-progress variables are denoted by Q and equilibrium constants by EQ.

The mnemonics for the input and output variable names in the subroutine calls are roughly the same as for the subroutine names.

In most cases the subroutines are backwards compatible with previous versions of Chemkin. However, there are a few cases where either the functionality is different or the call list has changed, but we have kept the same subroutine name. These are annotated with an asterisk(s).