diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index a4ac1fbc..a6ceeebd 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -42,8 +42,7 @@ Please make sure you follow the EasyScience organization-wide
 If you are not planning to contribute code, you may want to:
 
 - 🐞 Report a bug — see [Reporting Issues](#11-reporting-issues)
-- 🛡 Report a security issue — see
-  [Security Issues](#12-security-issues)
+- 🛡 Report a security issue — see [Security Issues](#12-security-issues)
 - 💬 Ask a question or start a discussion at
   [Project Discussions](https://github.com/easyscience/reflectometry-lib/discussions)
 
diff --git a/docs/docs/tutorials/simulation/bilayer.ipynb b/docs/docs/tutorials/simulation/bilayer.ipynb
index 5f37b010..987cd8e1 100644
--- a/docs/docs/tutorials/simulation/bilayer.ipynb
+++ b/docs/docs/tutorials/simulation/bilayer.ipynb
@@ -331,7 +331,7 @@
     "q = np.linspace(0.005, 0.3, 500)\n",
     "\n",
     "# Calculate reflectometry\n",
-    "reflectivity = model.interface().reflectity_profile(q, model.unique_name)\n",
+    "reflectivity = model.interface().reflectivity_profile(q, model.unique_name)\n",
     "\n",
     "# Plot\n",
     "plt.figure(figsize=(10, 6))\n",
@@ -391,7 +391,7 @@
    "outputs": [],
    "source": [
     "# First, compute reflectivity with current conformal roughness (3.0 Å)\n",
-    "reflectivity_conformal = model.interface().reflectity_profile(q, model.unique_name)\n",
+    "reflectivity_conformal = model.interface().reflectivity_profile(q, model.unique_name)\n",
     "\n",
     "# Disable conformal roughness to allow independent roughness per layer\n",
     "bilayer.conformal_roughness = False\n",
@@ -406,7 +406,7 @@
     "bilayer.back_head_layer.roughness.value = 4.0\n",
     "\n",
     "# Compute reflectivity with variable roughness\n",
-    "reflectivity_variable_roughness = model.interface().reflectity_profile(q, model.unique_name)\n",
+    "reflectivity_variable_roughness = model.interface().reflectivity_profile(q, model.unique_name)\n",
     "\n",
     "# Plot comparison\n",
     "plt.figure(figsize=(10, 6))\n",
@@ -629,8 +629,8 @@
    "outputs": [],
    "source": [
     "# Calculate reflectivity for both contrasts\n",
-    "reflectivity_d2o = model.interface().reflectity_profile(q, model.unique_name)\n",
-    "reflectivity_h2o = model_h2o.interface().reflectity_profile(q, model_h2o.unique_name)\n",
+    "reflectivity_d2o = model.interface().reflectivity_profile(q, model.unique_name)\n",
+    "reflectivity_h2o = model_h2o.interface().reflectivity_profile(q, model_h2o.unique_name)\n",
     "\n",
     "plt.figure(figsize=(10, 6))\n",
     "plt.semilogy(q, reflectivity_d2o, 'b-', linewidth=2, label='D₂O contrast')\n",
diff --git a/docs/docs/tutorials/simulation/magnetism.ipynb b/docs/docs/tutorials/simulation/magnetism.ipynb
index 8efdb9e4..ef226c41 100644
--- a/docs/docs/tutorials/simulation/magnetism.ipynb
+++ b/docs/docs/tutorials/simulation/magnetism.ipynb
@@ -252,7 +252,7 @@
     "model.resolution_function = PercentageFwhm(0)\n",
     "model_interface = model.interface()\n",
     "model_interface.magnetism = False\n",
-    "model_data_no_magnetism_ref1d_easy = model.interface().reflectity_profile(\n",
+    "model_data_no_magnetism_ref1d_easy = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
@@ -284,7 +284,7 @@
     "model.interface = interface\n",
     "model_interface = model.interface()\n",
     "model_interface.include_magnetism = True\n",
-    "model_data_magnetism = model.interface().reflectity_profile(\n",
+    "model_data_magnetism = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
@@ -301,7 +301,7 @@
     "model_interface._wrapper.update_layer(\n",
     "    list(model_interface._wrapper.storage['layer'].keys())[2], magnetism_rhoM=5, magnetism_thetaM=175\n",
     ")\n",
-    "model_data_magnetism_layer_1 = model.interface().reflectity_profile(\n",
+    "model_data_magnetism_layer_1 = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
@@ -365,7 +365,7 @@
     "model_interface._wrapper.update_layer(\n",
     "    list(model_interface._wrapper.storage['layer'].keys())[2], magnetism_rhoM=5, magnetism_thetaM=175\n",
     ")\n",
-    "model_data_magnetism_easy = model.interface().reflectity_profile(\n",
+    "model_data_magnetism_easy = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
@@ -478,7 +478,7 @@
     "interface.switch('refnx')\n",
     "model.interface = interface\n",
     "model_interface = model.interface()\n",
-    "model_data_no_magnetism_refnx = model.interface().reflectity_profile(\n",
+    "model_data_no_magnetism_refnx = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
@@ -488,7 +488,7 @@
     "interface.switch('refl1d')\n",
     "model.interface = interface\n",
     "model_interface = model.interface()\n",
-    "model_data_no_magnetism_ref1d = model.interface().reflectity_profile(\n",
+    "model_data_no_magnetism_ref1d = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
@@ -518,7 +518,7 @@
     "model.interface = interface\n",
     "model_interface = model.interface()\n",
     "model_interface.magnetism = True\n",
-    "model_data_magnetism = model.interface().reflectity_profile(\n",
+    "model_data_magnetism = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
@@ -529,7 +529,7 @@
     "model.interface = interface\n",
     "model_interface = model.interface()\n",
     "model_interface.magnetism = False\n",
-    "model_data_no_magnetism = model.interface().reflectity_profile(\n",
+    "model_data_no_magnetism = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
diff --git a/docs/docs/tutorials/simulation/resolution_functions.ipynb b/docs/docs/tutorials/simulation/resolution_functions.ipynb
index d46a76cb..cc7abe10 100644
--- a/docs/docs/tutorials/simulation/resolution_functions.ipynb
+++ b/docs/docs/tutorials/simulation/resolution_functions.ipynb
@@ -310,7 +310,7 @@
     "        num=1000,\n",
     "    )\n",
     "    model.resolution_function = resolution_function_dict[key]\n",
-    "    model_data = model.interface().reflectity_profile(\n",
+    "    model_data = model.interface().reflectivity_profile(\n",
     "        model_coords,\n",
     "        model.unique_name,\n",
     "    )\n",
@@ -363,14 +363,14 @@
     ")\n",
     "\n",
     "model.resolution_function = resolution_function_dict[key]\n",
-    "model_data = model.interface().reflectity_profile(\n",
+    "model_data = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
     "plt.plot(model_coords, model_data, 'k-', label='Variable', linewidth=5)\n",
     "\n",
     "model.resolution_function = PercentageFwhm(1.0)\n",
-    "model_data = model.interface().reflectity_profile(\n",
+    "model_data = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
@@ -402,7 +402,7 @@
     ")\n",
     "\n",
     "model.resolution_function = resolution_function_dict[key]\n",
-    "model_data = model.interface().reflectity_profile(\n",
+    "model_data = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
@@ -415,7 +415,7 @@
     "data_points.append(reference_data)  # R\n",
     "data_points.append(reference_variances)  # sQz (variance of Qz)\n",
     "model.resolution_function = Pointwise(q_data_points=data_points)\n",
-    "model_data = model.interface().reflectity_profile(\n",
+    "model_data = model.interface().reflectivity_profile(\n",
     "    model_coords,\n",
     "    model.unique_name,\n",
     ")\n",
diff --git a/docs/mkdocs.yml b/docs/mkdocs.yml
index f75e4657..1cd19cde 100644
--- a/docs/mkdocs.yml
+++ b/docs/mkdocs.yml
@@ -208,7 +208,8 @@ nav:
       - Elements:
           - Layers:
               - Layer: api-reference/elements/layer.md
-              - Layer Area Per Molecule: api-reference/elements/layer_area_per_molecule.md
+              - Layer Area Per Molecule:
+                  api-reference/elements/layer_area_per_molecule.md
           - Materials:
               - Material: api-reference/elements/material.md
               - Material Density: api-reference/elements/material_density.md
diff --git a/pixi.lock b/pixi.lock
index 6b3a38bc..983bc0bd 100644
--- a/pixi.lock
+++ b/pixi.lock
@@ -1,8 +1,21 @@
 version: 7
 platforms:
 - name: linux-64
-- name: osx-arm64
+  virtual-packages:
+  - __unix=0=0
+  - __linux=4.18
+  - __glibc=2.28
+  - __archspec=0=x86_64
+- name: p1
+  subdir: osx-arm64
+  virtual-packages:
+  - __osx=14.0
+  - __unix=0=0
+  - __archspec=0=m1
 - name: win-64
+  virtual-packages:
+  - __win=10.0
+  - __archspec=0=x86_64
 environments:
   default:
     channels:
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       - conda: https://conda.anaconda.org/conda-forge/noarch/zipp-3.23.1-pyhcf101f3_0.conda
-      - pypi: .
-      - pypi: git+https://github.com/easyscience/corelib?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
+      - pypi: ./
+      - pypi: git+https://github.com/easyscience/corelib.git?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
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@@ -326,7 +339,7 @@ environments:
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-      osx-arm64:
+      p1:
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       - conda: https://conda.anaconda.org/conda-forge/noarch/annotated-types-0.7.0-pyhd8ed1ab_1.conda
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       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/zstd-1.5.7-hbf9d68e_6.conda
-      - pypi: .
-      - pypi: git+https://github.com/easyscience/corelib?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
+      - pypi: ./
+      - pypi: git+https://github.com/easyscience/corelib.git?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
       - pypi: https://files.pythonhosted.org/packages/00/bb/90ba423612b6aa0adccc6b1874bcd4a9b44b660c0c16f346611e00f64ac3/backrefs-7.0-py313-none-any.whl
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-      - pypi: .
-      - pypi: git+https://github.com/easyscience/corelib?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
+      - pypi: ./
+      - pypi: git+https://github.com/easyscience/corelib.git?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
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-      - pypi: .
-      - pypi: git+https://github.com/easyscience/corelib?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
+      - pypi: ./
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-      osx-arm64:
+      p1:
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       - conda: https://conda.anaconda.org/conda-forge/noarch/anyio-4.13.0-pyhcf101f3_0.conda
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-      - pypi: .
-      - pypi: git+https://github.com/easyscience/corelib?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
+      - pypi: ./
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-      - pypi: git+https://github.com/easyscience/corelib?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
+      - pypi: ./
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@@ -2184,7 +2197,7 @@ environments:
       - pypi: https://files.pythonhosted.org/packages/fb/86/dd6e5db36df29e76c7a7699123569a4a18c1623ce68d826ed96c62643cae/mdit_py_plugins-0.5.0-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/fb/d9/8f612389178e1e1800aaed85537b024ebe28b9a82fff6a015825e51a8877/refl1d-1.0.1-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/fd/20/2870f2e63da2b74eda896053809065769b256646bb3f4b74e06d013ad590/uritools-6.1.0-py3-none-any.whl
-      osx-arm64:
+      p1:
       - conda: https://conda.anaconda.org/conda-forge/noarch/_python_abi3_support-1.0-hd8ed1ab_2.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/annotated-types-0.7.0-pyhd8ed1ab_1.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/anyio-4.13.0-pyhcf101f3_0.conda
@@ -2358,8 +2371,8 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/yaml-0.2.5-h925e9cb_3.conda
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/zeromq-4.3.5-h4818236_10.conda
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/zstd-1.5.7-hbf9d68e_6.conda
-      - pypi: .
-      - pypi: git+https://github.com/easyscience/corelib?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
+      - pypi: ./
+      - pypi: git+https://github.com/easyscience/corelib.git?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
       - pypi: https://files.pythonhosted.org/packages/00/bb/90ba423612b6aa0adccc6b1874bcd4a9b44b660c0c16f346611e00f64ac3/backrefs-7.0-py313-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/01/7c/fa07d3da2b6253eb8474be16eab2eadf670460e364ccc895ca7ff388ee30/oscrypto-1.3.0-py2.py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/04/11/432f32f8097b03e3cd5fe57e88efb685d964e2e5178a48ed61e841f7fdce/pyyaml_env_tag-1.1-py3-none-any.whl
@@ -2659,8 +2672,8 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/win-64/yaml-0.2.5-h6a83c73_3.conda
       - conda: https://conda.anaconda.org/conda-forge/win-64/zeromq-4.3.5-h507cc87_10.conda
       - conda: https://conda.anaconda.org/conda-forge/win-64/zstd-1.5.7-h534d264_6.conda
-      - pypi: .
-      - pypi: git+https://github.com/easyscience/corelib?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
+      - pypi: ./
+      - pypi: git+https://github.com/easyscience/corelib.git?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
       - pypi: https://files.pythonhosted.org/packages/00/bb/90ba423612b6aa0adccc6b1874bcd4a9b44b660c0c16f346611e00f64ac3/backrefs-7.0-py313-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/01/7c/fa07d3da2b6253eb8474be16eab2eadf670460e364ccc895ca7ff388ee30/oscrypto-1.3.0-py2.py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/04/11/432f32f8097b03e3cd5fe57e88efb685d964e2e5178a48ed61e841f7fdce/pyyaml_env_tag-1.1-py3-none-any.whl
@@ -3070,7 +3083,7 @@ environments:
       - pypi: https://files.pythonhosted.org/packages/f7/00/bbca25f8a2372465cdf93138c1e1e38dff045fb0afef1488f395d0afcb3b/ncrystal-4.4.2-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/fb/12/5911ae3eeec47800503a238d971e51722ccea5feb8569b735184d5fcdbc0/toolz-1.1.0-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/fb/76/641ae371508676492379f16e2fa48f4e2c11741bd63c48be4b12a6b09cba/aiosignal-1.4.0-py3-none-any.whl
-      osx-arm64:
+      p1:
       - conda: https://conda.anaconda.org/conda-forge/noarch/_python_abi3_support-1.0-hd8ed1ab_2.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/annotated-types-0.7.0-pyhd8ed1ab_1.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/anyio-4.13.0-pyhcf101f3_0.conda
@@ -8297,12 +8310,13 @@ packages:
   purls: []
   size: 388453
   timestamp: 1764777142545
-- pypi: .
+- pypi: ./
   name: easyreflectometry
   requires_dist:
   - bumps
-  - easyscience @ git+https://github.com/easyscience/corelib@develop
+  - easyscience @ git+https://github.com/easyscience/corelib.git@develop
   - orsopy
+  - plotly
   - pooch
   - refl1d>=1.0.0
   - refnx
@@ -8344,7 +8358,7 @@ packages:
   - validate-pyproject[all] ; extra == 'dev'
   - versioningit ; extra == 'dev'
   requires_python: '>=3.11'
-- pypi: git+https://github.com/easyscience/corelib?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
+- pypi: git+https://github.com/easyscience/corelib.git?rev=develop#aadbd4891b94f6aa18187d48be8c2ab6f81113b0
   name: easyscience
   version: 2.3.1+dev8
   requires_dist:
diff --git a/src/easyreflectometry/calculators/calculator_base.py b/src/easyreflectometry/calculators/calculator_base.py
index e2a92804..06fa5f07 100644
--- a/src/easyreflectometry/calculators/calculator_base.py
+++ b/src/easyreflectometry/calculators/calculator_base.py
@@ -189,7 +189,7 @@ def remove_item_from_model(self, item_id: str, model_id: str) -> None:
         """
         self._wrapper.remove_item(item_id, model_id)
 
-    def reflectity_profile(self, x_array: np.ndarray, model_id: str) -> np.ndarray:
+    def reflectivity_profile(self, x_array: np.ndarray, model_id: str) -> np.ndarray:
         """Determines the reflectivity profile for the given range and model.
 
         Parameters
diff --git a/src/easyreflectometry/calculators/factory.py b/src/easyreflectometry/calculators/factory.py
index c3e1479c..b1c1b170 100644
--- a/src/easyreflectometry/calculators/factory.py
+++ b/src/easyreflectometry/calculators/factory.py
@@ -40,6 +40,6 @@ def fit_func(self) -> Callable:
 
         def __fit_func(*args, **kwargs):
             """Fit func."""
-            return self().reflectity_profile(*args, **kwargs)
+            return self().reflectivity_profile(*args, **kwargs)
 
         return __fit_func
diff --git a/src/easyreflectometry/fitting.py b/src/easyreflectometry/fitting.py
index 83a3f6eb..3cb0ecb3 100644
--- a/src/easyreflectometry/fitting.py
+++ b/src/easyreflectometry/fitting.py
@@ -384,8 +384,8 @@ def mcmc_sample(
             — the population already exists in the saved state.
         :param progress_callback: Optional callback for progress updates during
             sampling.  Forwarded to the core MultiFitter.
-        :return: Dictionary with keys ``'draws'``, ``'param_names'``, ``'state'``,
-            and ``'logp'``.
+        :return: Dictionary with keys ``'draws'``, ``'param_names'``,
+            ``'internal_bumps_object'``, and ``'logp'``.
         :raises RuntimeError: If the current minimizer is not a BUMPS instance.
         """
         minimizer = self.easy_science_multi_fitter.minimizer
@@ -494,19 +494,3 @@ def switch_minimizer(self, minimizer: AvailableMinimizers) -> None:
             Minimizer to be switched to.
         """
         self.easy_science_multi_fitter.switch_minimizer(minimizer)
-
-
-def _flatten_list(this_list: list) -> list:
-    """Flatten nested lists.
-
-    Parameters
-    ----------
-    this_list : list
-        List to be flattened.
-
-    Returns
-    -------
-    list
-        Flattened list.
-    """
-    return np.array([item for sublist in this_list for item in sublist])
diff --git a/src/easyreflectometry/limits.py b/src/easyreflectometry/limits.py
index 001bba64..8962ab9b 100644
--- a/src/easyreflectometry/limits.py
+++ b/src/easyreflectometry/limits.py
@@ -31,14 +31,19 @@ def apply_default_limits(parameter: Parameter, kind: str) -> None:
 
 
 def _apply_percentage_limits(parameter: Parameter) -> None:
-    """Set min to 50% and max to 200% of the current value, only if current bounds are inf."""
+    """Set bounds to 50%-200% of the current value, only if current bounds are inf.
+
+    For negative values 0.5*value > 2*value, so the candidates are ordered
+    to keep min <= value <= max.
+    """
     value = parameter.value
     if value == 0.0:
         return
+    low, high = sorted((0.5 * value, 2.0 * value))
     if np.isinf(parameter.min):
-        parameter.min = 0.5 * value
+        parameter.min = low
     if np.isinf(parameter.max):
-        parameter.max = 2.0 * value
+        parameter.max = high
 
 
 def _apply_fixed_limits(parameter: Parameter, low: float, high: float) -> None:
diff --git a/src/easyreflectometry/model/model.py b/src/easyreflectometry/model/model.py
index da1396d0..5aa32833 100644
--- a/src/easyreflectometry/model/model.py
+++ b/src/easyreflectometry/model/model.py
@@ -148,12 +148,12 @@ def background(self, value: float) -> None:
 
     # ----- assembly management -----
 
-    def add_assemblies(self, *assemblies: list[BaseAssembly]) -> None:
+    def add_assemblies(self, *assemblies: BaseAssembly) -> None:
         """Add assemblies to the model sample.
 
         Parameters
         ----------
-        *assemblies : list[BaseAssembly]
+        *assemblies : BaseAssembly
             Assemblies to add to model sample.
         """
         if not assemblies:
diff --git a/src/easyreflectometry/model/resolution_functions.py b/src/easyreflectometry/model/resolution_functions.py
index fe5d2254..c723bf14 100644
--- a/src/easyreflectometry/model/resolution_functions.py
+++ b/src/easyreflectometry/model/resolution_functions.py
@@ -10,6 +10,7 @@
 
 from __future__ import annotations
 
+from abc import ABC
 from abc import abstractmethod
 from typing import List
 from typing import Optional
@@ -20,7 +21,7 @@
 DEFAULT_RESOLUTION_FWHM_PERCENTAGE = 5.0
 
 
-class ResolutionFunction:
+class ResolutionFunction(ABC):
     @abstractmethod
     def smearing(self, q: Union[np.array, float]) -> np.array: ...
 
diff --git a/src/easyreflectometry/project.py b/src/easyreflectometry/project.py
index 5ae272a6..fe4af1bf 100644
--- a/src/easyreflectometry/project.py
+++ b/src/easyreflectometry/project.py
@@ -346,29 +346,30 @@ def path_json(self):
         """Path json."""
         return self.path / 'project.json'
 
+    def _get_or_add_material_index(self, name: str, sld: float, isld: float) -> int:
+        """Return the index of the named material, adding it to the project
+        materials first if not present. This mutates ``self._materials``."""
+        names = [material.name for material in self._materials]
+        if name not in names:
+            self._materials.add_material(Material(name=name, sld=sld, isld=isld))
+            names.append(name)
+        return names.index(name)
+
     def get_index_air(self) -> int:
-        """Get index air."""
-        if 'Air' not in [material.name for material in self._materials]:
-            self._materials.add_material(Material(name='Air', sld=0.0, isld=0.0))
-        return [material.name for material in self._materials].index('Air')
+        """Index of the Air material, adding it to the project if missing."""
+        return self._get_or_add_material_index('Air', sld=0.0, isld=0.0)
 
     def get_index_si(self) -> int:
-        """Get index si."""
-        if 'Si' not in [material.name for material in self._materials]:
-            self._materials.add_material(Material(name='Si', sld=2.07, isld=0.0))
-        return [material.name for material in self._materials].index('Si')
+        """Index of the Si material, adding it to the project if missing."""
+        return self._get_or_add_material_index('Si', sld=2.07, isld=0.0)
 
     def get_index_sio2(self) -> int:
-        """Get index sio2."""
-        if 'SiO2' not in [material.name for material in self._materials]:
-            self._materials.add_material(Material(name='SiO2', sld=3.47, isld=0.0))
-        return [material.name for material in self._materials].index('SiO2')
+        """Index of the SiO2 material, adding it to the project if missing."""
+        return self._get_or_add_material_index('SiO2', sld=3.47, isld=0.0)
 
     def get_index_d2o(self) -> int:
-        """Get index d2o."""
-        if 'D2O' not in [material.name for material in self._materials]:
-            self._materials.add_material(Material(name='D2O', sld=6.36, isld=0.0))
-        return [material.name for material in self._materials].index('D2O')
+        """Index of the D2O material, adding it to the project if missing."""
+        return self._get_or_add_material_index('D2O', sld=6.36, isld=0.0)
 
     def load_orso_file(self, path: Union[Path, str]) -> None:
         """Load an ORSO file and optionally create a model and a data from it."""
@@ -386,7 +387,6 @@ def load_orso_file(self, path: Union[Path, str]) -> None:
             self._experiments[0].name = 'Experiment from ORSO'
             self._experiments[0].model = self.models[0]
             self._with_experiments = True
-        pass
 
     def set_sample_from_orso(self, sample: Sample) -> None:
         """Replace the current project model collection with a single model built from an ORSO-parsed sample.
@@ -665,7 +665,7 @@ def model_data_for_model_at_index(self, index: int = 0, q_range: Optional[np.arr
         if q_range is None:
             q_range = np.linspace(self.q_min, self.q_max, self.q_resolution)
         self.models[index].interface = self._calculator
-        reflectivity = self.models[index].interface().reflectity_profile(q_range, self._models[index].unique_name)
+        reflectivity = self.models[index].interface().reflectivity_profile(q_range, self._models[index].unique_name)
         return DataSet1D(
             name=f'Reflectivity for Model {index}',
             x=q_range,
@@ -868,10 +868,10 @@ def as_dict(self, include_materials_not_in_model=False):
             self._as_dict_add_materials_not_in_model_dict(project_dict)
         if self._with_experiments:
             self._as_dict_add_experiments(project_dict)
-        if self.fitter is not None:
-            project_dict['fitter_minimizer'] = self.fitter.easy_science_multi_fitter.minimizer.name
-        elif self._minimizer_selection is not None:
-            project_dict['fitter_minimizer'] = self._minimizer_selection.name
+        # Read the minimizer without touching the lazy `fitter` property:
+        # serialization must not construct a MultiFitter as a side effect.
+        if self.minimizer is not None:
+            project_dict['fitter_minimizer'] = self.minimizer.name
         if self._calculator is not None:
             project_dict['calculator'] = self._calculator.current_interface_name
         if self._colors is not None:
diff --git a/src/easyreflectometry/sample/elements/materials/material_mixture.py b/src/easyreflectometry/sample/elements/materials/material_mixture.py
index 1ab76ddc..b5b88e27 100644
--- a/src/easyreflectometry/sample/elements/materials/material_mixture.py
+++ b/src/easyreflectometry/sample/elements/materials/material_mixture.py
@@ -152,17 +152,16 @@ def fraction(self, value: float) -> None:
     # ----- derived sld / isld parameters (shared shape with Material) -----
     #
     # These are *derived* via the constraints set up in `_materials_constraints`
-    # (not constructor arguments) so we expose them as floats to match the
-    # legacy MaterialMixture API. The underlying Parameter objects remain
-    # available as `self._sld` / `self._isld`.
+    # (not constructor arguments), so unlike Material there are no setters:
+    # their values follow the child materials and the fraction.
 
     @property
-    def sld(self) -> float:
-        return self._sld.value
+    def sld(self) -> Parameter:
+        return self._sld
 
     @property
-    def isld(self) -> float:
-        return self._isld.value
+    def isld(self) -> Parameter:
+        return self._isld
 
     # ----- calculator binding -----
 
@@ -171,8 +170,8 @@ def _get_linkable_attributes(self):
 
         Override of the inherited `BaseCore._get_linkable_attributes`, which
         walks `get_all_variables()` and would otherwise expose the **child**
-        materials' sld/isld (because our own `sld` / `isld` are floats, not
-        Parameters). The calculator's `InterfaceFactoryTemplate.generate_bindings`
+        materials' sld/isld alongside the mixed ones. The calculator's
+        `InterfaceFactoryTemplate.generate_bindings`
         matches by parameter `name`; without this override it binds to
         `material_a.sld` and reflectivity is computed off the wrong SLD.
         """
diff --git a/src/easyreflectometry/special/calculations.py b/src/easyreflectometry/special/calculations.py
index 07844d3c..9c6e43c2 100644
--- a/src/easyreflectometry/special/calculations.py
+++ b/src/easyreflectometry/special/calculations.py
@@ -44,11 +44,13 @@ def neutron_scattering_length(formula: str) -> complex:
     scattering_length = 0 + 0j
     for key, value in formula_as_dict.items():
         scattering_length += pt.elements.symbol(key).neutron.b_c * value
+        # b_c_i is the imaginary (absorption) part of the bound coherent
+        # scattering length, not the incoherent scattering length.
         if pt.elements.symbol(key).neutron.b_c_i:
-            inc = pt.elements.symbol(key).neutron.b_c_i
+            imag = pt.elements.symbol(key).neutron.b_c_i
         else:
-            inc = 0
-        scattering_length += inc * 1j * value
+            imag = 0
+        scattering_length += imag * 1j * value
     return scattering_length * 1e-5
 
 
@@ -63,7 +65,7 @@ def molecular_weight(formula: str) -> float:
     Returns
     -------
     float
-        Molecular weight of the material in kilograms.
+        Molecular weight of the material in u (g/mol).
     """
     formula_as_dict = parse_formula(formula)
     mw = 0
diff --git a/tests/calculators/refl1d/test_refl1d_calculator.py b/tests/calculators/refl1d/test_refl1d_calculator.py
index 50de2db4..a27d1f7a 100644
--- a/tests/calculators/refl1d/test_refl1d_calculator.py
+++ b/tests/calculators/refl1d/test_refl1d_calculator.py
@@ -27,7 +27,7 @@ def test_init(self):
         assert_equal(p._model_link['background'], 'bkg')
         assert_equal(p.name, 'refl1d')
 
-    def test_reflectity_profile(self):
+    def test_reflectivity_profile(self):
         p = Refl1d()
         p._wrapper.create_material('Material1')
         p._wrapper.update_material('Material1', rho=0.000, irho=0.000)
@@ -63,7 +63,7 @@ def test_reflectity_profile(self):
             1.3093e-07,
             1.0520e-07,
         ]
-        assert_almost_equal(p.reflectity_profile(q, 'MyModel'), expected, decimal=4)
+        assert_almost_equal(p.reflectivity_profile(q, 'MyModel'), expected, decimal=4)
 
     def test_calculate2(self):
         p = Refl1d()
@@ -95,7 +95,7 @@ def test_calculate2(self):
         p._wrapper.add_item('Item3', 'MyModel')
         p._wrapper.update_item('Item2', repeat=10)
         q = np.linspace(0.001, 0.3, 10)
-        actual = p.reflectity_profile(q, 'MyModel')
+        actual = p.reflectivity_profile(q, 'MyModel')
         expected = [
             9.9949e-01,
             8.7414e-03,
@@ -140,7 +140,7 @@ def test_calculate_magnetic(self):
         p._wrapper.add_item('Item2', 'MyModel')
         p._wrapper.add_item('Item3', 'MyModel')
         q = np.linspace(0.001, 0.3, 10)
-        actual = p.reflectity_profile(q, 'MyModel')
+        actual = p.reflectivity_profile(q, 'MyModel')
         expected = [
             9.99491251e-01,
             1.08413641e-02,
diff --git a/tests/calculators/refnx/test_refnx_calculator.py b/tests/calculators/refnx/test_refnx_calculator.py
index baeb9296..e4efae68 100644
--- a/tests/calculators/refnx/test_refnx_calculator.py
+++ b/tests/calculators/refnx/test_refnx_calculator.py
@@ -27,7 +27,7 @@ def test_init(self):
         assert_equal(p._model_link['background'], 'bkg')
         assert_equal(p.name, 'refnx')
 
-    def test_reflectity_profile(self):
+    def test_reflectivity_profile(self):
         p = Refnx()
         p._wrapper.create_material('Material1')
         p._wrapper.update_material('Material1', real=0.000, imag=0.000)
@@ -62,7 +62,7 @@ def test_reflectity_profile(self):
             1.26726993e-07,
             1.01842852e-07,
         ]
-        assert_almost_equal(p.reflectity_profile(q, 'MyModel'), expected)
+        assert_almost_equal(p.reflectivity_profile(q, 'MyModel'), expected)
 
     def test_calculate2(self):
         p = Refnx()
@@ -105,7 +105,7 @@ def test_calculate2(self):
             3.4981523e-07,
             2.5424356e-07,
         ]
-        assert_almost_equal(p.reflectity_profile(q, 'MyModel'), expected)
+        assert_almost_equal(p.reflectivity_profile(q, 'MyModel'), expected)
 
     def test_sld_profile(self):
         p = Refnx()
diff --git a/tests/model/test_model.py b/tests/model/test_model.py
index b11149c1..0c83d5dc 100644
--- a/tests/model/test_model.py
+++ b/tests/model/test_model.py
@@ -427,8 +427,8 @@ def test_dict_round_trip(interface):
     if interface is not None:
         assert model.interface().name == model_from_dict.interface().name
         assert_almost_equal(
-            model.interface().reflectity_profile([0.3], model.unique_name),
-            model_from_dict.interface().reflectity_profile([0.3], model_from_dict.unique_name),
+            model.interface().reflectivity_profile([0.3], model.unique_name),
+            model_from_dict.interface().reflectivity_profile([0.3], model_from_dict.unique_name),
         )
 
 
diff --git a/tests/model/test_resolution_functions.py b/tests/model/test_resolution_functions.py
index b8a4ea18..1391949b 100644
--- a/tests/model/test_resolution_functions.py
+++ b/tests/model/test_resolution_functions.py
@@ -38,7 +38,7 @@ def test_as_dict(self):
         resolution_function = PercentageFwhm(1.0)
 
         # Then Expect
-        resolution_function.as_dict() == {'smearing': 'PercentageFwhm', 'constant': 1.0}
+        assert resolution_function.as_dict() == {'smearing': 'PercentageFwhm', 'constant': 1.0}
 
     def test_dict_round_trip(self):
         # When
@@ -67,7 +67,7 @@ def test_as_dict(self):
         resolution_function = LinearSpline(q_data_points=[0, 10], fwhm_values=[5, 10])
 
         # Then Expect
-        resolution_function.as_dict() == {
+        assert resolution_function.as_dict() == {
             'smearing': 'LinearSpline',
             'q_data_points': [0, 10],
             'fwhm_values': [5, 10],
@@ -117,7 +117,12 @@ def test_as_dict(self):
         resolution_function = Pointwise(q_data_points=self.data_points)
 
         # Then Expect
-        assert resolution_function.as_dict(), {'smearing': 'Pointwise', 'q_data_points': [0, 10]}
+        assert resolution_function.as_dict() == {
+            'smearing': 'Pointwise',
+            'q_data_points': self.data_points[0],
+            'R_data_points': self.data_points[1],
+            'sQz_data_points': self.data_points[2],
+        }
 
     def test_dict_round_trip(self):
         # When
diff --git a/tests/sample/elements/layers/test_layer_area_per_molecule.py b/tests/sample/elements/layers/test_layer_area_per_molecule.py
index 0d466be7..7fe04009 100644
--- a/tests/sample/elements/layers/test_layer_area_per_molecule.py
+++ b/tests/sample/elements/layers/test_layer_area_per_molecule.py
@@ -27,8 +27,8 @@ def test_default(self):
         assert p.roughness.value == 3.3
         assert str(p.roughness.unit) == 'Å'
         assert p.roughness.fixed is True
-        assert_almost_equal(p.material.sld, 2.268770124481328)
-        assert_almost_equal(p.material.isld, 0)
+        assert_almost_equal(p.material.sld.value, 2.268770124481328)
+        assert_almost_equal(p.material.isld.value, 0)
         assert p.material.name == 'C10H18NO8P in D2O'
         assert p.solvent.sld.value == 6.36
         assert p.solvent.isld.value == 0
@@ -69,7 +69,7 @@ def test_from_pars_constraint(self):
         )
         assert p.molecular_formula == 'C8O10H12P'
         assert p.area_per_molecule.value == 50
-        assert_almost_equal(p.material.sld, 0.31494833333333333)
+        assert_almost_equal(p.material.sld.value, 0.31494833333333333)
         assert p.thickness.value == 12
         assert p.roughness.value == 2
         assert p.solvent.sld.value == -0.561
@@ -77,10 +77,10 @@ def test_from_pars_constraint(self):
         assert p.solvent_fraction.value == 0.5
         p.area_per_molecule = 30
         assert p.area_per_molecule.value == 30
-        assert_almost_equal(p.material.sld, 0.7119138888888887)
+        assert_almost_equal(p.material.sld.value, 0.7119138888888887)
         p.thickness.value = 10
         assert p.thickness.value == 10
-        assert_almost_equal(p.material.sld, 0.9103966666666665)
+        assert_almost_equal(p.material.sld.value, 0.9103966666666665)
 
     @unittest.skip('Instantiation of LayerAreaPerMolecule fails, despite working everywhere else.')
     def test_solvent_change(self):
@@ -97,7 +97,7 @@ def test_solvent_change(self):
         assert p.molecular_formula == 'C8O10H12P'
         assert p.area_per_molecule.value == 50
         print(p.material)
-        assert_almost_equal(p.material.sld, 0.31494833333333333)
+        assert_almost_equal(p.material.sld.value, 0.31494833333333333)
         assert p.thickness.value == 12
         assert p.roughness.value == 2
         assert p.solvent.sld.value == -0.561
@@ -107,7 +107,7 @@ def test_solvent_change(self):
         p.solvent = d2o
         assert p.molecular_formula == 'C8O10H12P'
         assert p.area_per_molecule.value == 50
-        assert_almost_equal(p.material.sld, 3.762948333333333)
+        assert_almost_equal(p.material.sld.value, 3.762948333333333)
         assert p.thickness.value == 12
         assert p.roughness.value == 2
         assert p.solvent.sld.value == 6.335
@@ -127,7 +127,7 @@ def test_molecular_formula_change(self):
         )
         assert p.molecular_formula == 'C8O10H12P'
         assert p.area_per_molecule.value == 50
-        assert_almost_equal(p.material.sld, 0.31494833333333333)
+        assert_almost_equal(p.material.sld.value, 0.31494833333333333)
         assert p.thickness.value == 12
         assert p.roughness.value == 2
 
@@ -138,7 +138,7 @@ def test_molecular_formula_change(self):
         p.molecular_formula = 'C8O10D12P'
         assert p.molecular_formula == 'C8O10D12P'
         assert p.area_per_molecule.value == 50
-        assert_almost_equal(p.material.sld, 1.3558483333333333)
+        assert_almost_equal(p.material.sld.value, 1.3558483333333333)
         assert p.thickness.value == 12
         assert p.roughness.value == 2
         assert p.solvent.sld.value == -0.561
diff --git a/tests/sample/elements/materials/test_material_mixture.py b/tests/sample/elements/materials/test_material_mixture.py
index 3c2a3f65..d3912b7d 100644
--- a/tests/sample/elements/materials/test_material_mixture.py
+++ b/tests/sample/elements/materials/test_material_mixture.py
@@ -15,8 +15,8 @@ def test_default(self) -> None:
         material_mixture = MaterialMixture()
         assert material_mixture.fraction.value == 0.5
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 4.186)
-        assert_almost_equal(material_mixture.isld, 0)
+        assert_almost_equal(material_mixture.sld.value, 4.186)
+        assert_almost_equal(material_mixture.isld.value, 0)
         assert str(material_mixture._sld.unit) == '1/Å^2'
         assert str(material_mixture._isld.unit) == '1/Å^2'
 
@@ -24,12 +24,12 @@ def test_default_constraint(self) -> None:
         material_mixture = MaterialMixture()
         assert material_mixture.fraction.value == 0.5
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 4.186)
-        assert_almost_equal(material_mixture.isld, 0)
+        assert_almost_equal(material_mixture.sld.value, 4.186)
+        assert_almost_equal(material_mixture.isld.value, 0)
         material_mixture.material_a.sld.value = 0
         material_mixture.material_b.isld.value = -1
-        assert_almost_equal(material_mixture.sld, 2.093)
-        assert_almost_equal(material_mixture.isld, -0.5)
+        assert_almost_equal(material_mixture.sld.value, 2.093)
+        assert_almost_equal(material_mixture.isld.value, -0.5)
         assert str(material_mixture._sld.unit) == '1/Å^2'
         assert str(material_mixture._isld.unit) == '1/Å^2'
 
@@ -38,59 +38,59 @@ def test_fraction_constraint(self):
         q = Material(6.908, -0.278, 'Boron')
         material_mixture = MaterialMixture(p, q, 0.2)
         assert material_mixture.fraction.value == 0.2
-        assert_almost_equal(material_mixture.sld, 4.7304)
-        assert_almost_equal(material_mixture.isld, -0.0556)
+        assert_almost_equal(material_mixture.sld.value, 4.7304)
+        assert_almost_equal(material_mixture.isld.value, -0.0556)
         material_mixture._fraction.value = 0.5
         assert material_mixture.fraction.value == 0.5
-        assert_almost_equal(material_mixture.sld, 5.54700)
-        assert_almost_equal(material_mixture.isld, -0.1390)
+        assert_almost_equal(material_mixture.sld.value, 5.54700)
+        assert_almost_equal(material_mixture.isld.value, -0.1390)
 
     def test_material_a_change(self) -> None:
         material_mixture = MaterialMixture()
         assert material_mixture.fraction.value == 0.5
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 4.186)
-        assert_almost_equal(material_mixture.isld, 0)
+        assert_almost_equal(material_mixture.sld.value, 4.186)
+        assert_almost_equal(material_mixture.isld.value, 0)
         q = Material(6.908, -0.278, 'Boron')
         material_mixture.material_a = q
         assert material_mixture.fraction.value == 0.5
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 5.54700)
-        assert_almost_equal(material_mixture.isld, -0.1390)
+        assert_almost_equal(material_mixture.sld.value, 5.54700)
+        assert_almost_equal(material_mixture.isld.value, -0.1390)
 
     def test_material_b_change(self) -> None:
         material_mixture = MaterialMixture()
         assert material_mixture.fraction.value == 0.5
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 4.186)
-        assert_almost_equal(material_mixture.isld, 0)
+        assert_almost_equal(material_mixture.sld.value, 4.186)
+        assert_almost_equal(material_mixture.isld.value, 0)
         q = Material(6.908, -0.278, 'Boron')
         material_mixture.material_b = q
         assert material_mixture.fraction.value == 0.5
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 5.54700)
-        assert_almost_equal(material_mixture.isld, -0.1390)
+        assert_almost_equal(material_mixture.sld.value, 5.54700)
+        assert_almost_equal(material_mixture.isld.value, -0.1390)
 
     def test_material_b_change_double(self) -> None:
         material_mixture = MaterialMixture()
         assert material_mixture.fraction.value == 0.5
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 4.186)
-        assert_almost_equal(material_mixture.isld, 0)
+        assert_almost_equal(material_mixture.sld.value, 4.186)
+        assert_almost_equal(material_mixture.isld.value, 0)
         q = Material(6.908, -0.278, 'Boron')
         material_mixture.material_b = q
         assert material_mixture.name == 'EasyMaterial/Boron'
         assert material_mixture.fraction.value == 0.5
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 5.54700)
-        assert_almost_equal(material_mixture.isld, -0.1390)
+        assert_almost_equal(material_mixture.sld.value, 5.54700)
+        assert_almost_equal(material_mixture.isld.value, -0.1390)
         r = Material(0.00, 0.00, 'ACMW')
         material_mixture.material_b = r
         assert material_mixture.name == 'EasyMaterial/ACMW'
         assert material_mixture.fraction.value == 0.5
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 2.0930)
-        assert_almost_equal(material_mixture.isld, 0.0000)
+        assert_almost_equal(material_mixture.sld.value, 2.0930)
+        assert_almost_equal(material_mixture.isld.value, 0.0000)
 
     def test_from_pars(self):
         p = Material()
@@ -98,8 +98,8 @@ def test_from_pars(self):
         material_mixture = MaterialMixture(p, q, 0.2)
         assert material_mixture.fraction.value == 0.2
         assert str(material_mixture._fraction.unit) == 'dimensionless'
-        assert_almost_equal(material_mixture.sld, 4.7304)
-        assert_almost_equal(material_mixture.isld, -0.0556)
+        assert_almost_equal(material_mixture.sld.value, 4.7304)
+        assert_almost_equal(material_mixture.isld.value, -0.0556)
         assert str(material_mixture._sld.unit) == '1/Å^2'
         assert str(material_mixture._isld.unit) == '1/Å^2'
 
@@ -158,7 +158,7 @@ def test_calculator_binding_uses_mixed_sld(self) -> None:
         mixture = MaterialMixture(material_a, material_b, fraction=0.25, interface=interface)
 
         # 2 * 0.75 + 6 * 0.25 = 1.5 + 1.5 = 3.0
-        assert_almost_equal(mixture.sld, 3.0)
+        assert_almost_equal(mixture.sld.value, 3.0)
         wrapper_material = interface()._wrapper.storage['material'][mixture.unique_name]
         assert_almost_equal(wrapper_material.real.value, 3.0)
         assert_almost_equal(wrapper_material.imag.value, 0.0)
@@ -173,8 +173,8 @@ def test_mutation_propagates_after_round_trip(self) -> None:
         global_object.map._clear()
 
         q = MaterialMixture.from_dict(p_dict)
-        assert_almost_equal(q.sld, 3.0)
+        assert_almost_equal(q.sld.value, 3.0)
 
         q.fraction = 0.8
         # 2 * 0.2 + 6 * 0.8 = 0.4 + 4.8 = 5.2
-        assert_almost_equal(q.sld, 5.2)
+        assert_almost_equal(q.sld.value, 5.2)
diff --git a/tests/test_project.py b/tests/test_project.py
index ec85b3f2..b1ae72eb 100644
--- a/tests/test_project.py
+++ b/tests/test_project.py
@@ -476,7 +476,7 @@ def test_as_dict_minimizer(self):
         project = Project()
         project._fitter = MagicMock()
         project._fitter.easy_science_multi_fitter = MagicMock()
-        project._fitter.easy_science_multi_fitter.minimizer = AvailableMinimizers.LMFit
+        project._fitter.easy_science_multi_fitter.minimizer.enum = AvailableMinimizers.LMFit
 
         # Then
         project_dict = project.as_dict()
diff --git a/tests/test_topmost_nesting.py b/tests/test_topmost_nesting.py
index 622991b2..1003efe5 100644
--- a/tests/test_topmost_nesting.py
+++ b/tests/test_topmost_nesting.py
@@ -47,8 +47,8 @@ def test_copy():
     assert model._resolution_function.smearing(5.5) == model_copy._resolution_function.smearing(5.5)
     assert model.interface().name == model_copy.interface().name
     assert_almost_equal(
-        model.interface().reflectity_profile([0.3], model.unique_name),
-        model_copy.interface().reflectity_profile([0.3], model_copy.unique_name),
+        model.interface().reflectivity_profile([0.3], model.unique_name),
+        model_copy.interface().reflectivity_profile([0.3], model_copy.unique_name),
     )
     assert model.unique_name != model_copy.unique_name
     assert model.name == model_copy.name