# Clarification on the Subdomain Formulation for F.Reaction

**URL:** <https://festim.discourse.group/t/clarification-on-the-subdomain-formulation-for-f-reaction/176>\
**Category:** User Support\
**Tags:** festim2\
**Created:** [July 20, 2026, 8:26pm UTC](https://festim.discourse.group/t/clarification-on-the-subdomain-formulation-for-f-reaction/176 "2026-07-20T20:26:08Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![ee-nn](https://yyz2.discourse-cdn.com/free1/user_avatar/festim.discourse.group/ee-nn/32/6_2.png) [@ee-nn](https://festim.discourse.group/u/ee-nn)\
**Post date:** [July 20, 2026, 8:26pm UTC](https://festim.discourse.group/t/clarification-on-the-subdomain-formulation-for-f-reaction/176/1 "2026-07-20T20:26:08Z")

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Hi everyone,

I have a quick question on the `Reaction` class, upon which `Trap` is based. It appears that when assigning the subdomain, it explicitly imports the 1D subclass of `VolumeSubdomain` ([link](https://github.com/festim-dev/FESTIM/blob/c2c17d54202e295486ea63f6d0e8d9b4c74417e4/src/festim/reaction.py#L9)):

```python
from festim.subdomain.volume_subdomain import VolumeSubdomain1D as VS1D

```

However, the 1D version just appears to be a copy of the the main `VolumeSubdmain`, but with a `borders` initiated ([link](https://github.com/festim-dev/FESTIM/blob/c2c17d54202e295486ea63f6d0e8d9b4c74417e4/src/festim/subdomain/volume_subdomain.py#L112)):

```python
class VolumeSubdomain1D(VolumeSubdomain):
    """Volume subdomain class for 1D cases.

    Args:
        id (int): the id of the volume subdomain
        borders (list of float): the borders of the volume subdomain
        material (festim.Material): the material of the volume subdomain

    Attributes:
        id (int): the id of the volume subdomain
        borders (list of float): the borders of the volume subdomain
        material (festim.Material): the material of the volume subdomain

    Examples:

        .. testsetup:: VolumeSubdomain1D

            from festim import VolumeSubdomain1D, Material
            my_mat = Material(D_0=1, E_D=1, name="test_mat")

        .. testcode:: VolumeSubdomain1D

            VolumeSubdomain1D(id=1, borders=[0, 1], material=my_mat)
    """

    def __init__ (self, id, borders, material) -> None:
        super(). __init__ (
            id,
            material,
            locator=lambda x: np.logical_and(x[0] >= borders[0], x[0] <= borders[1]),
        )
        self.borders = borders

```

Therefore, I’m curious if the 1D subclass is actually being used for anything?

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**Author:** ![jhdark](https://yyz2.discourse-cdn.com/free1/user_avatar/festim.discourse.group/jhdark/32/13_2.png) [@jhdark](https://festim.discourse.group/u/jhdark)\
**Post date:** [July 20, 2026, 8:47pm UTC](https://festim.discourse.group/t/clarification-on-the-subdomain-formulation-for-f-reaction/176/2 "2026-07-20T20:47:52Z")

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Hey @ee-nn

So seems you’ve found a mistake there! So the line `from festim.subdomain.volume_subdomain import VolumeSubdomain1D as VS1D` is only bringing in the volume subdomain for type hinting, however, this is incorrect as the reaction class does accept any dimension of the subdomain and should be using the standard `VolumeSubdomain`.

The 1D version of that class does need to exist, though, as when doing a multi-material system, the users need to define the borders of the different materials/subdomains, and we have some checks in place to make sure all the borders cover the whole domain and that every cell is covered.

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<div class="post-metadata">

**Author:** ![ee-nn](https://yyz2.discourse-cdn.com/free1/user_avatar/festim.discourse.group/ee-nn/32/6_2.png) [@ee-nn](https://festim.discourse.group/u/ee-nn)\
**Post date:** [July 20, 2026, 10:10pm UTC](https://festim.discourse.group/t/clarification-on-the-subdomain-formulation-for-f-reaction/176/3 "2026-07-20T22:10:21Z")

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Hi @jhdark thanks for the clarification! I can open an issue on the repo.