# Types of boundary conditions

“Which types of boundary conditions does Morpheus provide?”

Morpheus provides three types of boundary conditions.

Name | Synonym | Meaning |
---|---|---|

Periodic |
Wrap-around | Lattice is wrapped around to opposite site, such that there is no border. |

Constant |
Dirichlet | Border states have specified constant values. |

Noflux |
Neumann | Derivative at border is zero. |

These can be specified in `Space`

→ `Lattice`

→ `BoundaryConditions`

. These define the structure of the boundaries.

Note that there are six boundaries (*x*,*-x*, *y*, *-y*, *z*, *-z*,). All need to be specified, expect when periodic boundaries are used (in which case *+x=-x*, *+y=-y*, *+z=-z*).

## Define boundary condition

To define boundaries, add entries in `Space`

→ `Lattice`

→ `BoundaryConditions`

.

## Define boundary values

To specify the values the boundaries should take, we distinguish between Fields (i.e. Reaction-Diffusion models) and cell-based models. `Periodic`

boundaries do not use boundary values, since these are topologically connected.

### Fields (Reaction-Diffusion models, PDE)

When using `constant`

boundary conditions, values used at the boundary of a reaction-diffusion model can be specified as expressions in `Field`

→ `BoundaryValue`

. Assuming non-zero diffusion, this causes a flux across the boundary depending on the actual concentration gradient at that boundary.

The default value is *0.0*.

When using `noflux`

boundary conditions, the value plays no role in reactions-diffusion systems, since the flux is set to zero. However, the `NeighborhoodReporter`

accesses these boundary values when computing mappings of the local neighborhood.

### Cell-based models

Any *medium* cell type can be specified as `noflux`

or `constant`

boundary. First, define a cell type of type `medium`

under `CellTypes`

. To specify this at the boundary of a cell-based model with `constant`

boundary conditions, use `CellPopulatons`

→ `BoundaryValue`

.

**Note** that cells interact with the choosen boundary type, but only in the case of `constant`

boundary the boundary cell type may extend / flow into the simulation space.