diff --git a/metatomic-torch/include/metatomic/torch/model_output.hpp b/metatomic-torch/include/metatomic/torch/model_output.hpp
deleted file mode 100644
index e69de29b..00000000
diff --git a/metatomic-torch/src/model.cpp b/metatomic-torch/src/model.cpp
index efa77c9c..95a41016 100644
--- a/metatomic-torch/src/model.cpp
+++ b/metatomic-torch/src/model.cpp
@@ -1206,6 +1206,24 @@ static std::map<std::string, Quantity> KNOWN_QUANTITIES = {
     }, {
         // alternative names
     }}},
+    {"mass", Quantity{/* name */ "mass", /* baseline */ "u ", {
+        {"u", 1.0},
+        {"kilogram", 1.66053906892e-27},
+        {"gram", 1.66053906892e-24},
+    }, {
+        // alternative names
+        {"Dalton", "u"},
+        {"kg", "kilogram"},
+        {"g", "gram"},
+    }}},
+    {"velocity", Quantity{/* name */ "velocity", /* baseline */ "nm/fs", {
+        {"nm/fs", 1.0},
+        {"A/fs", 1e1},
+        {"m/s", 1e6},
+        {"nm/ps", 1e3},
+    }, {
+        // alternative names
+    }}}
 };
 
 bool metatomic_torch::valid_quantity(const std::string& quantity) {
diff --git a/metatomic-torch/tests/models.cpp b/metatomic-torch/tests/models.cpp
index 3401c23f..ec8996f1 100644
--- a/metatomic-torch/tests/models.cpp
+++ b/metatomic-torch/tests/models.cpp
@@ -109,7 +109,7 @@ TEST_CASE("Models metadata") {
         struct WarningHandler: public torch::WarningHandler {
             virtual ~WarningHandler() override = default;
             void process(const torch::Warning& warning) override {
-                CHECK(warning.msg() == "unknown quantity 'unknown', only [energy force length momentum pressure] are supported");
+                CHECK(warning.msg() == "unknown quantity 'unknown', only [energy force length mass momentum pressure velocity] are supported");
             }
         };
 
diff --git a/python/metatomic_torch/metatomic/torch/ase_calculator.py b/python/metatomic_torch/metatomic/torch/ase_calculator.py
index 7c04989f..d6891ec6 100644
--- a/python/metatomic_torch/metatomic/torch/ase_calculator.py
+++ b/python/metatomic_torch/metatomic/torch/ase_calculator.py
@@ -21,6 +21,7 @@
     pick_device,
     pick_output,
     register_autograd_neighbors,
+    unit_conversion_factor,
 )
 
 
@@ -43,6 +44,28 @@
     "float64": torch.float64,
 }
 
+ARRAY_QUANTITIES = {
+    "momentum": {
+        "getter": lambda atoms: atoms.get_momenta(),
+        "unit": "(eV*u)^(1/2)",
+    },
+    "mass": {
+        "getter": lambda atoms: atoms.get_masses(),
+        "unit": "u",
+    },
+    "velocity": {
+        "getter": lambda atoms: atoms.get_velocities(),
+        "unit": "nm/fs",
+    },
+    "initial_magmoms": {},
+    "magmom": {},
+    "magmoms": {},
+    "initial_charges": {},
+    "charges": {},
+    "dipole": {},
+    "free_energy": {},
+}
+
 
 class MetatomicCalculator(ase.calculators.calculator.Calculator):
     """
@@ -339,6 +362,12 @@ def run_model(
                     check_consistency=self.parameters["check_consistency"],
                 )
                 system.add_neighbor_list(options, neighbors)
+            # Get the additional inputs requested by the model
+            for quantity, option in self._model.requested_inputs().items():
+                input_tensormap = _get_ase_input(
+                    atoms, option, dtype=self._dtype, device=self._device
+                )
+                system.add_data(quantity, input_tensormap)
             systems.append(system)
 
         available_outputs = self._model.capabilities().outputs
@@ -475,7 +504,6 @@ def calculate(
         with record_function("MetatomicCalculator::compute_neighbors"):
             # convert from ase.Atoms to metatomic.torch.System
             system = System(types, positions, cell, pbc)
-
             for options in self._model.requested_neighbor_lists():
                 neighbors = _compute_ase_neighbors(
                     atoms, options, dtype=self._dtype, device=self._device
@@ -486,6 +514,11 @@ def calculate(
                     check_consistency=self.parameters["check_consistency"],
                 )
                 system.add_neighbor_list(options, neighbors)
+            for quantity, option in self._model.requested_inputs().items():
+                input_tensormap = _get_ase_input(
+                    atoms, option, dtype=self._dtype, device=self._device
+                )
+                system.add_data(quantity, input_tensormap)
 
         # no `record_function` here, this will be handled by AtomisticModel
         outputs = self._model(
@@ -906,6 +939,56 @@ def _compute_ase_neighbors(atoms, options, dtype, device):
     )
 
 
+def _get_ase_input(
+    atoms: ase.Atoms,
+    option: ModelOutput,
+    dtype: torch.dtype,
+    device: torch.device,
+) -> "TensorMap":
+    if option.quantity in ARRAY_QUANTITIES:
+        if len(ARRAY_QUANTITIES[option.quantity]) == 0:
+            raise NotImplementedError(
+                f"Though the quantity {option.quantity} is available in `ase`, it is "
+                "currently not supported by metatomic."
+            )
+        infos = ARRAY_QUANTITIES[option.quantity]
+    else:
+        raise ValueError(
+            f"The model requested '{option.quantity}', which is not available in `ase`."
+        )
+
+    values = infos["getter"](atoms)
+    if infos["unit"] != option.unit:
+        conversion = unit_conversion_factor(
+            option.quantity,
+            from_unit=infos["unit"],
+            to_unit=option.unit,
+        )
+    else:
+        conversion = 1.0
+    values = (
+        torch.tensor(values[:, :, None] if values.ndim == 2 else values[None, :, None])
+        * conversion
+    )
+
+    tblock = TensorBlock(
+        values,
+        samples=Labels.range("atoms", values.shape[0]),
+        components=[Labels.range("components", values.shape[1])]
+        if values.shape[1] != 1
+        else [],
+        properties=Labels([option.quantity], torch.tensor([[0]])),
+    )
+    tmap = TensorMap(
+        Labels(["_"], torch.tensor([[0]])),
+        [tblock],
+    )
+    tmap.set_info("quantity", option.quantity)
+    tmap.set_info("unit", option.unit)
+    tmap.to(dtype=dtype, device=device)
+    return tmap
+
+
 def _ase_to_torch_data(atoms, dtype, device):
     """Get the positions, cell and pbc from ASE atoms as torch tensors"""
 
diff --git a/python/metatomic_torch/metatomic/torch/model.py b/python/metatomic_torch/metatomic/torch/model.py
index eb1c2703..db209d4a 100644
--- a/python/metatomic_torch/metatomic/torch/model.py
+++ b/python/metatomic_torch/metatomic/torch/model.py
@@ -176,6 +176,11 @@ def requested_neighbor_lists(self) -> List[NeighborListOptions]:
            the systems before calling the model.
         """
 
+    def requested_inputs(self) -> Dict[str, ModelOutput]:
+        """
+        Optional method declaring which inputs this model requires.
+        """
+
 
 class AtomisticModel(torch.nn.Module):
     """
@@ -292,6 +297,7 @@ class AtomisticModel(torch.nn.Module):
 
     # Some annotation to make the TorchScript compiler happy
     _requested_neighbor_lists: List[NeighborListOptions]
+    _requested_inputs: Dict[str, ModelOutput]
 
     def __init__(
         self,
@@ -327,6 +333,15 @@ def __init__(
         )
         # ============================================================================ #
 
+        # recursively explore `module` to get all the requested_additional_inputs
+        self._requested_inputs = {}
+        _get_requested_inputs(
+            module,
+            self.module.__class__.__name__,
+            self._requested_inputs,
+        )
+        # ============================================================================ #
+
         self._metadata = metadata
         self._capabilities = capabilities
 
@@ -380,6 +395,14 @@ def requested_neighbor_lists(self) -> List[NeighborListOptions]:
         """
         return self._requested_neighbor_lists
 
+    @torch.jit.export
+    def requested_inputs(self) -> Dict[str, ModelOutput]:
+        """
+        Get the additional inputs required by the exported model or any of the child
+        module.
+        """
+        return self._requested_inputs
+
     def forward(
         self,
         systems: List[System],
@@ -410,10 +433,23 @@ def forward(
                 _check_inputs(
                     capabilities=self._capabilities,
                     requested_neighbor_lists=self._requested_neighbor_lists,
+                    requested_inputs=self._requested_inputs,
                     systems=systems,
                     options=options,
                     expected_dtype=self._model_dtype,
                 )
+                # check the requested inputs stored in the `systems`
+                for system in systems:
+                    system_inputs: Dict[str, TensorMap] = {}
+                    for name in system.known_data():
+                        system_inputs[name] = system.get_data(name)
+                    _check_outputs(
+                        systems=[system],
+                        requested=self._requested_inputs,
+                        selected_atoms=options.selected_atoms,
+                        outputs=system_inputs,
+                        model_dtype=self._capabilities.dtype,
+                    )
 
         with record_function("AtomisticModel::check_atomic_types"):
             # always (i.e. even if check_consistency=False) check that the atomic types
@@ -622,6 +658,30 @@ def _get_requested_neighbor_lists(
         )
 
 
+def _get_requested_inputs(
+    module: torch.nn.Module,
+    module_name: str,
+    requested: Dict[str, ModelOutput],
+):
+    if hasattr(module, "requested_inputs"):
+        requested_inputs = module.requested_inputs()
+        for new_options in requested_inputs:
+            already_requested = False
+            for existing in requested:
+                if existing == new_options:
+                    already_requested = True
+
+            if not already_requested:
+                requested[new_options] = requested_inputs[new_options]
+
+    for child_name, child in module.named_children():
+        _get_requested_inputs(
+            module=child,
+            module_name=module_name + "." + child_name,
+            requested=requested,
+        )
+
+
 def _check_annotation(module: torch.nn.Module):
     if isinstance(module, torch.jit.RecursiveScriptModule):
         _check_annotation_torchscript(module)
@@ -750,6 +810,7 @@ def _check_annotation_python(module: torch.nn.Module):
 def _check_inputs(
     capabilities: ModelCapabilities,
     requested_neighbor_lists: List[NeighborListOptions],
+    requested_inputs: Dict[str, ModelOutput],
     systems: List[System],
     options: ModelEvaluationOptions,
     expected_dtype: torch.dtype,
@@ -848,6 +909,23 @@ def _check_inputs(
                     "in the system"
                 )
 
+        # Check additional inputs
+        # Might be problematic, this requires that only requested inputs are stored as
+        # the data pf the system
+        known_additional_inputs = system.known_data()
+        for request in requested_inputs:
+            found = False
+            for known in known_additional_inputs:
+                if request == known:
+                    found = True
+
+            if not found:
+                raise ValueError(
+                    "missing additional input in the system: the model requested "
+                    f"a list for {request}, but it was not computed and stored "
+                    "in the system"
+                )
+
 
 def _convert_systems_units(
     systems: List[System],
diff --git a/python/metatomic_torch/tests/ase_calculator.py b/python/metatomic_torch/tests/ase_calculator.py
index e654a579..96e8e21d 100644
--- a/python/metatomic_torch/tests/ase_calculator.py
+++ b/python/metatomic_torch/tests/ase_calculator.py
@@ -14,6 +14,7 @@
 import pytest
 import torch
 from ase.calculators.calculator import PropertyNotImplementedError
+from ase.md.velocitydistribution import MaxwellBoltzmannDistribution
 from metatensor.torch import Labels, TensorBlock, TensorMap
 
 from metatomic.torch import (
@@ -25,6 +26,7 @@
     System,
 )
 from metatomic.torch.ase_calculator import (
+    ARRAY_QUANTITIES,
     MetatomicCalculator,
     _compute_ase_neighbors,
     _full_3x3_to_voigt_6_stress,
@@ -809,3 +811,57 @@ def forward(
     match = "does not support energy computation"
     with pytest.raises(ValueError, match=match):
         calc.compute_energy(atoms)
+
+
+class AdditionalInputModel(torch.nn.Module):
+    def __init__(self, inputs):
+        super().__init__()
+        self._requested_inputs = inputs
+
+    def requested_inputs(self) -> List[str]:
+        return self._requested_inputs
+
+    def forward(
+        self,
+        systems: List[System],
+        outputs: Dict[str, ModelOutput],
+        selected_atoms: Optional[Labels] = None,
+    ) -> Dict[str, TensorMap]:
+        return {
+            ("extra::" + input): systems[0].get_data(input)
+            for input in self._requested_inputs
+        }
+
+
+def test_additional_input(atoms):
+    inputs = {
+        "mass": ModelOutput(quantity="mass", unit="u", per_atom=True),
+        "velocity": ModelOutput(quantity="velocity", unit="A/fs", per_atom=True),
+    }
+    outputs = {("extra::" + prop): inputs[prop] for prop in inputs}
+    capabilities = ModelCapabilities(
+        outputs=outputs,
+        atomic_types=[28],
+        interaction_range=0.0,
+        supported_devices=["cpu"],
+        dtype="float64",
+    )
+
+    model = AtomisticModel(
+        AdditionalInputModel(inputs).eval(), ModelMetadata(), capabilities
+    )
+    MaxwellBoltzmannDistribution(atoms, temperature_K=300.0)
+    calculator = MetatomicCalculator(model)
+    results = calculator.run_model(atoms, outputs)
+    for k, v in results.items():
+        head, prop = k.split("::")
+        assert head == "extra"
+        assert prop in inputs
+        assert len(v.keys.names) == 1
+        assert v.get_info("quantity") == inputs[prop].quantity
+        shape = v[0].values.numpy().shape
+        assert np.allclose(
+            v[0].values.numpy(),
+            ARRAY_QUANTITIES[prop]["getter"](atoms).reshape(shape)
+            * (10 if prop == "velocity" else 1),
+        )