diff --git a/docs/src/architectures/soap-bpnn.rst b/docs/src/architectures/soap-bpnn.rst
index 1c87eda3f..f0715081b 100644
--- a/docs/src/architectures/soap-bpnn.rst
+++ b/docs/src/architectures/soap-bpnn.rst
@@ -147,10 +147,15 @@ The parameters for training are
 :param learning_rate: learning rate
 :param log_interval: number of epochs that elapse between reporting new training results
 :param checkpoint_interval: Interval to save a checkpoint to disk.
-:param per_atom_targets: Specifies whether the model should be trained on a per-atom
+:param fixed_composition_weights: allows to set fixed isolated atom energies from
+    outside. These are per target name and per (integer) atom type. For example,
+    ``fixed_composition_weights: {"energy": {1: -396.0, 6: -500.0}, "mtt::U0": {1: 0.0,
+    6: 0.0}}`` sets the isolated atom energies for H (``1``) and O (``6``) to different
+    values for the two distinct targets.
+:param per_atom_targets: specifies whether the model should be trained on a per-atom
     loss. In that case, the logger will also output per-atom metrics for that target. In
     any case, the final summary will be per-structure.
-:param loss_weights: Specifies the weights to be used in the loss for each target. The
+:param loss_weights: specifies the weights to be used in the loss for each target. The
     weights should be a dictionary of floats, one for each target. All missing targets
     are assigned a weight of 1.0.
 
diff --git a/src/metatrain/experimental/soap_bpnn/model.py b/src/metatrain/experimental/soap_bpnn/model.py
index 54980c0b6..a2be10d10 100644
--- a/src/metatrain/experimental/soap_bpnn/model.py
+++ b/src/metatrain/experimental/soap_bpnn/model.py
@@ -144,7 +144,7 @@ def __init__(self, model_hypers: Dict, dataset_info: DatasetInfo) -> None:
             * (self.hypers["soap"]["max_angular"] + 1)
         )
 
-        hypers_bpnn = self.hypers["bpnn"]
+        hypers_bpnn = {**self.hypers["bpnn"]}
         hypers_bpnn["input_size"] = soap_size
 
         if hypers_bpnn["layernorm"]:
diff --git a/src/metatrain/experimental/soap_bpnn/schema-hypers.json b/src/metatrain/experimental/soap_bpnn/schema-hypers.json
index 2ce8a1b4e..570931d49 100644
--- a/src/metatrain/experimental/soap_bpnn/schema-hypers.json
+++ b/src/metatrain/experimental/soap_bpnn/schema-hypers.json
@@ -120,8 +120,14 @@
         "fixed_composition_weights": {
           "type": "object",
           "patternProperties": {
-            "^[0-9]+$": {
-              "type": "number"
+            "^.*$": {
+              "type": "object",
+              "propertyNames": {
+                "pattern": "^[0-9]+$"
+              },
+              "additionalProperties": {
+                "type": "number"
+              }
             }
           },
           "additionalProperties": false
diff --git a/src/metatrain/experimental/soap_bpnn/tests/test_functionality.py b/src/metatrain/experimental/soap_bpnn/tests/test_functionality.py
index 45dae1a8d..25dd250b6 100644
--- a/src/metatrain/experimental/soap_bpnn/tests/test_functionality.py
+++ b/src/metatrain/experimental/soap_bpnn/tests/test_functionality.py
@@ -1,11 +1,15 @@
 import metatensor.torch
+import pytest
 import torch
+from jsonschema.exceptions import ValidationError
 from metatensor.torch.atomistic import ModelOutput, System
+from omegaconf import OmegaConf
 
 from metatrain.experimental.soap_bpnn import SoapBpnn
+from metatrain.utils.architectures import check_architecture_options
 from metatrain.utils.data import DatasetInfo, TargetInfo, TargetInfoDict
 
-from . import MODEL_HYPERS
+from . import DEFAULT_HYPERS, MODEL_HYPERS
 
 
 def test_prediction_subset_elements():
@@ -195,3 +199,33 @@ def test_output_per_atom():
 
     assert outputs["energy"].block().samples.names == ["system", "atom"]
     assert outputs["energy"].block().values.shape == (4, 1)
+
+
+def test_fixed_composition_weights():
+    """Tests the correctness of the json schema for fixed_composition_weights"""
+
+    hypers = DEFAULT_HYPERS.copy()
+    hypers["training"]["fixed_composition_weights"] = {
+        "energy": {
+            1: 1.0,
+            6: 0.0,
+            7: 0.0,
+            8: 0.0,
+            9: 3000.0,
+        }
+    }
+    hypers = OmegaConf.create(hypers)
+    check_architecture_options(
+        name="experimental.soap_bpnn", options=OmegaConf.to_container(hypers)
+    )
+
+
+def test_fixed_composition_weights_error():
+    """Test that only inputd of type Dict[str, Dict[int, float]] are allowed."""
+    hypers = DEFAULT_HYPERS.copy()
+    hypers["training"]["fixed_composition_weights"] = {"energy": {"H": 300.0}}
+    hypers = OmegaConf.create(hypers)
+    with pytest.raises(ValidationError, match=r"'H' does not match '\^\[0-9\]\+\$'"):
+        check_architecture_options(
+            name="experimental.soap_bpnn", options=OmegaConf.to_container(hypers)
+        )