# Exterior computational domain

The **exterior computational domain** refers to the size of the total computational region and the applied boundary conditions.

## Total computational region

The **computational region**, irrespective of other simulation details, is **always** cuboid in shape, with dimensions specified by the simulation parameter domain size. See the image to the right.

As an example, suppose that one wants to change the size of the all dimensions to 200 nm (from the default value of 100 nm). In this case, one could add the following to the parameters file:
```
```

```
```domain size 200.0e-9

```
One could also explicitly set the sizes along each dimension independently:
```

```
```

```
```domain size 200.0e-9 x
domain size 200.0e-9 y
domain size 200.0e-9 z

```
The latter example demonstrates how one could define non-cubic
```

**exterior computational domains**.

## Boundary conditions

**Boundary conditions** are simulation-dependent. See here for the types of **boundary conditions** supported in FDTD++, and how to define them.