# 4.3.4. Initial Conditions

This section documents the initial condition, IC, line commands within the current version of Aria. The IC simply represents an initialization of the Field variable. Technically speaking initial conditions have meaning only in time-dependent partial differential equations. However, within the Aria code the IC command line results in an initialization of Field values used in the assembly of a Jacobian matrix. Thus the IC is equally applicable for assigning starting values in a steady solution.

Within the input file initial conditions command lines will appear at the Region scope as illustrated below. When problem physics within the Region are defined using Equation Systems, initial conditions must be defined outside the scope of the Equation Systems. Note that input specification of initial conditions on portions of the FEM mesh can be simplified by aggregation of element blocks into assemblies and mesh groups.

The general format of an initial condition line command is as follows:

    IC for [DOF] on [PART] = [MODEL] [MODEL ARGS...]

Here the `DOF` must be one associated with an equation declared with an `EQ` line at either `Aria Region` or `Equation System` scope (see Equation Specification). Note that material phase, species, or LS phase can be specified in the `DOF` argument e.g.

    # IC for gas phase temperature
    IC for temperature in gas_phase on block_1 = ...

    # IC for Li+ in liquid phase
    IC for species of Li+ in liquid_phase on block_1 = ...

See General Naming Convention for more information.

The `PART` corresponds to a mesh entity, and can include both single parts (e.g. block_1) as well as assemblies or mesh groups (see Assemblies and Mesh Groups).

Finally the `MODEL` defines how the source term is calculated. Note that multiple source lines may be defined for a given `DOF` / `PART`.

For example, a constant IC for temperature can be defined as follows

    # Scope: Sierra > Procedure > Aria Region
    IC for temperature on block_1 = constant value = 300 # K

The reader is referred to the command reference for a comprehensive list of all possible initial condition commands.