Elements and Species

A description of how elements and species are defined in YAML input files

Elements

Cantera provides built-in definitions for the chemical elements, including values for their atomic weights taken from IUPAC / CIAAW. These elements can be used by speciying the corresponding atomic symbols when specifying the composition of species.

In order to give a name to a particular isotope or a virtual element representing a surface site, a custom element entry can be used. The default location for element entries is the elements section of the input file. Elements defined in this section will automatically be considered for addition to phases defined in the same file. Elements can be defined in other sections of the input file if those sections are named explicitly in the elements field of the phase definition.

An element entry has the following fields:

  • symbol: The symbol to be used for the element, for example when specifying the composition of a species.

  • atomic-weight: The atomic weight of the element, in unified atomic mass units (dalton)

  • atomic-number: The atomic number of the element. Optional.

  • entropy298: The standard molar entropy of the element at 298.15 K. Optional.

An example elements section:

elements:
- symbol: C13
  atomic-weight: 13.003354826
  atomic-number: 12
- symbol: O-18
  atomic-weight: 17.9991603

Species

A species entry in Cantera is used to specify the name, composition, thermodynamic, and transport properties of an individual species.

The default location for species entries is in the species section of the input file. Species defined in this section will automatically be considered for addition to phases defined in the same file. Species can be defined in other sections of the input file or in other input files, and these species definitions can be used in phase definitions by explicitly referencing the section name.

Species Name

The name of a species is given in the name field of a species entry. Names may include almost all printable characters, with the exception of spaces. The use of some characters such as [, ], and , may require that species names be enclosed in quotes when written in YAML. Some valid species names given in a YAML list include:

[CH4, methane, argon_2+, "C[CH2]", CH2(singlet), "H2O,l"]

Elemental Composition

The elemental composition of a species is specified as a mapping in the composition entry.

For gaseous species, the elemental composition is well-defined, since the species represent distinct molecules. For species in solid or liquid solutions, or on surfaces, there may be several possible ways of defining the species. For example, an aqueous species might be defined with or without including the water molecules in the solvation cage surrounding it.

For surface species, it is possible for the composition mapping to be empty, in which case the species is composed of nothing, and represents an empty surface site. This can also be done to represent vacancies in solids. A charged vacancy can be defined to be composed solely of electrons.

The number of atoms of an element must be non-negative, except for the special "element" E that represents an electron.

Examples:

composition: {C: 1, O: 2}  # carbon dioxide
composition: {Ar: 1, E: -2}  # Ar++
composition: {Y: 1, Ba: 2, Cu: 3, O: 6.5}  # stoichiometric YBCO
composition: {}  # A surface species representing an empty site

Thermodynamic Properties

In addition to the thermodynamic model used at the phase level for computing properties, parameterizations are usually required for the enthalpy, entropy, and specific heat capacities of individual species under standard conditions. These parameterizations are provided in the thermo field of each species entry.

The parameterization used to provide this information is specified by the model field of the thermo field. The models available are:

  • NASA7: 7-coefficient NASA polynomials in one or two temperature regions

  • NASA9: 9-coefficient NASA polynomials in one or more temperature regions

  • Shomate: Shomate polynomials in one or two temperature regions

  • constant-cp: Constant heat capacity

  • piecewise-Gibbs: Interpolation between tabulated Gibbs free energies using a constant heat capacity in each temperature interval

The fields used by each model are described and examples provided in the linked documentation.

Species Equation of State

For some phase thermodynamic models, additional equation of state parameterizations are needed for each species. This information is provided in the equation-of-state field of each species entry, with the type of parameterization used specified by the model field of the equation-of-state field. The models available are:

The fields used by each model are described and examples provided in the linked documentation.

Species Transport Coefficients

Transport-related parameters for each species are needed in order to calculate transport properties of a phase. These parameters are provided in the transport field of each species entry, with the type of the parameterization used specified by the model field of the transport field. The only model type specifically handled is gas. The parameters used depend on the transport model specified at the phase level. The full set of possible parameters is described in the API documentation.

An example of a transport entry:

transport:
  model: gas
  geometry: linear
  well-depth: 107.4
  diameter: 3.458
  polarizability: 1.6
  rotational-relaxation: 3.8