not all systems have atoms, e.g. electron gas

netobs/systems/__init__.py

@@ -14,11 +14,6 @@
 
 from typing import TypedDict
 
-from jax import numpy as jnp
-
 
 class System(TypedDict):
-    atoms: jnp.ndarray
-    "(natom, ndim) Positions of the atoms."
-
     ndim: int

netobs/systems/elec_gas.py

@@ -0,0 +1,19 @@
+# Copyright 2022-2024 Bytedance Ltd. and/or its affiliates
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from netobs.systems import System
+
+
+class ElectronGas(System):
+    spins: tuple[int, int]

netobs/systems/molecule.py

@@ -18,6 +18,9 @@
 
 
 class MolecularSystem(System):
+    atoms: jnp.ndarray
+    "(natom, ndim) Positions of the atoms."
+
     charges: jnp.ndarray
     "(natom,) Nuclear charges of the atoms."
 

netobs/systems/solid.py

@@ -23,6 +23,9 @@
 
 
 class SolidSystem(System):
+    atoms: jnp.ndarray
+    "(natom, ndim) Positions of the atoms."
+
     charges: jnp.ndarray
     "(natom,) Nuclear charges of the atoms."