diff --git a/pybamm/expression_tree/binary_operators.py b/pybamm/expression_tree/binary_operators.py
index 8eeeded17..abffe9c4c 100644
--- a/pybamm/expression_tree/binary_operators.py
+++ b/pybamm/expression_tree/binary_operators.py
@@ -112,12 +112,15 @@ class BinaryOperator(pybamm.Symbol):
 
     def new_copy(self):
         """ See :meth:`pybamm.Symbol.new_copy()`. """
+
         # process children
         new_left = self.left.new_copy()
         new_right = self.right.new_copy()
+
         # make new symbol, ensure domain remains the same
         out = self.__class__(new_left, new_right)
         out.domain = self.domain
+
         return out
 
     def evaluate(self, t=None, y=None, known_evals=None):
diff --git a/pybamm/expression_tree/simplify.py b/pybamm/expression_tree/simplify.py
index 1d285bd71..45f5ec5ea 100644
--- a/pybamm/expression_tree/simplify.py
+++ b/pybamm/expression_tree/simplify.py
@@ -70,6 +70,9 @@ def simplify_addition_subtraction(myclass, left, right):
 
         outputs to lists `numerator` and `numerator_types`
 
+        Note that domains are all set to [] as we do not wish to consider domains once
+        simplifications are applied
+
         e.g.
 
         (1 + 2) + 3       -> [1, 2, 3]    and [None, Addition, Addition]
@@ -77,6 +80,9 @@ def simplify_addition_subtraction(myclass, left, right):
         1 - (2 + 3)       -> [1, 2, 3]    and [None, Subtraction, Subtraction]
         (1 + 2) - (2 + 3) -> [1, 2, 2, 3] and [None, Addition, Subtraction, Subtraction]
         """
+
+        left_child.domain = []
+        right_child.domain = []
         for side, child in [("left", left_child), ("right", right_child)]:
             if isinstance(child, (pybamm.Addition, pybamm.Subtraction)):
                 left, right = child.orphans
@@ -264,6 +270,9 @@ def simplify_multiplication_division(myclass, left, right):
         Note that multiplication *within* matrix multiplications, e.g. a@(b*c), are not
         flattened into a@b*c, as this would be incorrect (see #253)
 
+        Note that the domains are all set to [] as we do not wish to consider domains
+        once simplifications are applied
+
         outputs to lists `numerator`, `denominator` and `numerator_types`
 
         e.g.
@@ -272,6 +281,9 @@ def simplify_multiplication_division(myclass, left, right):
         (1 @ 2) / 3 ->  [1, 2]       [3]       [None, MatrixMultiplication]
         1 / (c / 2) ->  [1, 2]       [c]       [None, Multiplication]
         """
+
+        left_child.domain = []
+        right_child.domain = []
         for side, child in [("left", left_child), ("right", right_child)]:
 
             if side == "left":
diff --git a/pybamm/expression_tree/vector.py b/pybamm/expression_tree/vector.py
index be46f4c06..0aee6aad2 100644
--- a/pybamm/expression_tree/vector.py
+++ b/pybamm/expression_tree/vector.py
@@ -141,7 +141,7 @@ class StateVector(pybamm.Symbol):
 
     def new_copy(self):
         """ See :meth:`pybamm.Symbol.new_copy()`. """
-        return StateVector(self.y_slice, self.name, domain=[])
+        return StateVector(self.y_slice, self.name, domain=self.domain)
 
     def evaluate_for_shape(self):
         """

divide outer product
doing Division(Division(Outer, exp), exp) fails for some expressions

**To Reproduce**
see `test_simplify_divide_outer` in `test_simplify.py` [here](https://github.com/pybamm-team/PyBaMM/blob/c9d94496b3c62f23aec82165b8ff7d422151329b/tests/unit/test_expression_tree/test_simplify.py#L543) which implements the following:

```
    u = pybamm.Scalar(1)
    v = pybamm.StateVector(slice(0, 5), domain="current collector")
    outer = pybamm.Outer(v, u)

    exp1 = pybamm.Division(pybamm.Division(outer, u), u)
    self.assertIsInstance(exp1.simplify(), pybamm.Outer)

    exp2 = pybamm.Division(pybamm.Division(outer, 2 * u), u)
    self.assertIsInstance(exp2.simplify(), pybamm.Multiplication)

    exp3 = pybamm.Division(pybamm.Division(outer, u), 2 * u)
    self.assertIsInstance(exp3.simplify(), pybamm.Multiplication)

    exp4 = pybamm.Division(pybamm.Division(outer, 2 * u), 2 * u)
    exp4.simplify()
```
The first three expressions simplify as expected, but exp4 gives 

`pybamm.expression_tree.exceptions.DomainError: left child domain must be 'current collector', not'[]'`

diff --git a/tests/unit/test_expression_tree/test_simplify.py b/tests/unit/test_expression_tree/test_simplify.py
index 61ebe1657..66a5cdcdf 100644
--- a/tests/unit/test_expression_tree/test_simplify.py
+++ b/tests/unit/test_expression_tree/test_simplify.py
@@ -540,6 +540,23 @@ class TestSimplify(unittest.TestCase):
         self.assertIsInstance(outer_simp, pybamm.Vector)
         np.testing.assert_array_equal(outer_simp.evaluate(), 2 * np.ones((15, 1)))
 
+    def test_simplify_divide_outer(self):
+        u = pybamm.Scalar(1)
+        v = pybamm.StateVector(slice(0, 5), domain="current collector")
+        outer = pybamm.Outer(v, u)
+
+        exp1 = pybamm.Division(pybamm.Division(outer, u), u)
+        self.assertIsInstance(exp1.simplify(), pybamm.Outer)
+
+        exp2 = pybamm.Division(pybamm.Division(outer, 2 * u), u)
+        self.assertIsInstance(exp2.simplify(), pybamm.Multiplication)
+
+        exp3 = pybamm.Division(pybamm.Division(outer, u), 2 * u)
+        self.assertIsInstance(exp3.simplify(), pybamm.Multiplication)
+
+        exp4 = pybamm.Division(pybamm.Division(outer, 2 * u), 2 * u)
+        exp4.simplify()
+
 
 if __name__ == "__main__":
     print("Add -v for more debug output")

pip install -e .[dev,docs]
parse_log_pytest
requirements.txt
pytest flake8
apt-get update apt-get install -y gfortran gcc libopenblas-dev liblapack-dev graphviz
3.6
requirements/base.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.13
anyio==3.6.2
anytree==2.8.0
argon2-cffi==21.3.0
argon2-cffi-bindings==21.2.0
async-generator==1.10
attrs==22.2.0
autograd==1.6.2
Babel==2.11.0
backcall==0.2.0
bleach==4.1.0
certifi==2021.5.30
cffi==1.15.1
charset-normalizer==2.0.12
comm==0.1.4
contextvars==2.4
cycler==0.11.0
dataclasses==0.8
decorator==5.1.1
defusedxml==0.7.1
docutils==0.17.1
entrypoints==0.4
flake8==5.0.4
future==1.0.0
guzzle-sphinx-theme==0.7.11
idna==3.10
imagesize==1.4.1
immutables==0.19
importlib-metadata==4.2.0
iniconfig==1.1.1
ipykernel==5.5.6
ipython==7.16.3
ipython-genutils==0.2.0
ipywidgets==7.8.5
jedi==0.17.2
Jinja2==3.0.3
json5==0.9.16
jsonschema==3.2.0
jupyter==1.1.1
jupyter-client==7.1.2
jupyter-console==6.4.3
jupyter-core==4.9.2
jupyter-server==1.13.1
jupyterlab==3.2.9
jupyterlab-pygments==0.1.2
jupyterlab-server==2.10.3
jupyterlab_widgets==1.1.11
kiwisolver==1.3.1
MarkupSafe==2.0.1
matplotlib==3.3.4
mccabe==0.7.0
mistune==0.8.4
nbclassic==0.3.5
nbclient==0.5.9
nbconvert==6.0.7
nbformat==5.1.3
nest-asyncio==1.6.0
notebook==6.4.10
numpy==1.19.5
packaging==21.3
pandas==1.1.5
pandocfilters==1.5.1
parso==0.7.1
pexpect==4.9.0
pickleshare==0.7.5
Pillow==8.4.0
pluggy==1.0.0
prometheus-client==0.17.1
prompt-toolkit==3.0.36
ptyprocess==0.7.0
py==1.11.0
-e git+https://github.com/pybamm-team/PyBaMM.git@af3bd87346215d68b87e27dff2e9131bfe507d90#egg=pybamm
pycodestyle==2.9.1
pycparser==2.21
pyflakes==2.5.0
Pygments==2.14.0
pyparsing==3.1.4
pyrsistent==0.18.0
pytest==7.0.1
python-dateutil==2.9.0.post0
pytz==2025.2
pyzmq==25.1.2
requests==2.27.1
scipy==1.5.4
Send2Trash==1.8.3
six==1.17.0
sniffio==1.2.0
snowballstemmer==2.2.0
Sphinx==4.3.2
sphinxcontrib-applehelp==1.0.2
sphinxcontrib-devhelp==1.0.2
sphinxcontrib-htmlhelp==2.0.0
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==1.0.3
sphinxcontrib-serializinghtml==1.1.5
terminado==0.12.1
testpath==0.6.0
tomli==1.2.3
tornado==6.1
traitlets==4.3.3
typing_extensions==4.1.1
urllib3==1.26.20
wcwidth==0.2.13
webencodings==0.5.1
websocket-client==1.3.1
widgetsnbextension==3.6.10
zipp==3.6.0

name: PyBaMM
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2021.5.30=py36h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - pip=21.2.2=py36h06a4308_0
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.13
      - anyio==3.6.2
      - anytree==2.8.0
      - argon2-cffi==21.3.0
      - argon2-cffi-bindings==21.2.0
      - async-generator==1.10
      - attrs==22.2.0
      - autograd==1.6.2
      - babel==2.11.0
      - backcall==0.2.0
      - bleach==4.1.0
      - cffi==1.15.1
      - charset-normalizer==2.0.12
      - comm==0.1.4
      - contextvars==2.4
      - cycler==0.11.0
      - dataclasses==0.8
      - decorator==5.1.1
      - defusedxml==0.7.1
      - docutils==0.17.1
      - entrypoints==0.4
      - flake8==5.0.4
      - future==1.0.0
      - guzzle-sphinx-theme==0.7.11
      - idna==3.10
      - imagesize==1.4.1
      - immutables==0.19
      - importlib-metadata==4.2.0
      - iniconfig==1.1.1
      - ipykernel==5.5.6
      - ipython==7.16.3
      - ipython-genutils==0.2.0
      - ipywidgets==7.8.5
      - jedi==0.17.2
      - jinja2==3.0.3
      - json5==0.9.16
      - jsonschema==3.2.0
      - jupyter==1.1.1
      - jupyter-client==7.1.2
      - jupyter-console==6.4.3
      - jupyter-core==4.9.2
      - jupyter-server==1.13.1
      - jupyterlab==3.2.9
      - jupyterlab-pygments==0.1.2
      - jupyterlab-server==2.10.3
      - jupyterlab-widgets==1.1.11
      - kiwisolver==1.3.1
      - markupsafe==2.0.1
      - matplotlib==3.3.4
      - mccabe==0.7.0
      - mistune==0.8.4
      - nbclassic==0.3.5
      - nbclient==0.5.9
      - nbconvert==6.0.7
      - nbformat==5.1.3
      - nest-asyncio==1.6.0
      - notebook==6.4.10
      - numpy==1.19.5
      - packaging==21.3
      - pandas==1.1.5
      - pandocfilters==1.5.1
      - parso==0.7.1
      - pexpect==4.9.0
      - pickleshare==0.7.5
      - pillow==8.4.0
      - pluggy==1.0.0
      - prometheus-client==0.17.1
      - prompt-toolkit==3.0.36
      - ptyprocess==0.7.0
      - py==1.11.0
      - pycodestyle==2.9.1
      - pycparser==2.21
      - pyflakes==2.5.0
      - pygments==2.14.0
      - pyparsing==3.1.4
      - pyrsistent==0.18.0
      - pytest==7.0.1
      - python-dateutil==2.9.0.post0
      - pytz==2025.2
      - pyzmq==25.1.2
      - requests==2.27.1
      - scipy==1.5.4
      - send2trash==1.8.3
      - six==1.17.0
      - sniffio==1.2.0
      - snowballstemmer==2.2.0
      - sphinx==4.3.2
      - sphinxcontrib-applehelp==1.0.2
      - sphinxcontrib-devhelp==1.0.2
      - sphinxcontrib-htmlhelp==2.0.0
      - sphinxcontrib-jsmath==1.0.1
      - sphinxcontrib-qthelp==1.0.3
      - sphinxcontrib-serializinghtml==1.1.5
      - terminado==0.12.1
      - testpath==0.6.0
      - tomli==1.2.3
      - tornado==6.1
      - traitlets==4.3.3
      - typing-extensions==4.1.1
      - urllib3==1.26.20
      - wcwidth==0.2.13
      - webencodings==0.5.1
      - websocket-client==1.3.1
      - widgetsnbextension==3.6.10
      - zipp==3.6.0
prefix: /opt/conda/envs/PyBaMM

tests/unit/test_expression_tree/test_simplify.py::TestSimplify::test_simplify_divide_outer
tests/unit/test_expression_tree/test_simplify.py::test_const_function tests/unit/test_expression_tree/test_simplify.py::TestSimplify::test_domain_concatenation_simplify tests/unit/test_expression_tree/test_simplify.py::TestSimplify::test_function_simplify tests/unit/test_expression_tree/test_simplify.py::TestSimplify::test_matrix_simplifications tests/unit/test_expression_tree/test_simplify.py::TestSimplify::test_simplify_concatenation_state_vectors tests/unit/test_expression_tree/test_simplify.py::TestSimplify::test_simplify_outer tests/unit/test_expression_tree/test_simplify.py::TestSimplify::test_symbol_simplify tests/unit/test_expression_tree/test_simplify.py::TestSimplify::test_vector_zero_simplify
pybamm/expression_tree/binary_operators.py pybamm/expression_tree/vector.py pybamm/expression_tree/simplify.py
pybamm/expression_tree/binary_operators.py pybamm/expression_tree/vector.py pybamm/expression_tree/simplify.py