diff --git a/tests/test_trajectory.py b/tests/test_trajectory.py
index 67d9de1f..47206e10 100644
--- a/tests/test_trajectory.py
+++ b/tests/test_trajectory.py
@@ -260,7 +260,7 @@ def test_data_type_conversions(test_file: Path) -> None:
 
     rng = np.random.default_rng(seed=0)
     # Test data with different types
-    test_data = {
+    test_data: dict[str, np.ndarray | torch.Tensor] = {
         # NumPy arrays
         "np_float64": rng.random((10, 3)).astype(np.float64),
         "np_float32": rng.random((10, 3)).astype(np.float32),
@@ -376,7 +376,7 @@ def test_scalar_dtype_handling(test_file: Path) -> None:
         mode="w",
     )
 
-    scalar_data = {
+    scalar_data: dict[str, np.ndarray | np.generic | torch.Tensor] = {
         "float64_scalar": np.float64(1.0),
         "float32_scalar": np.float32(1.0),
         "int64_scalar": np.int64(1),
diff --git a/torch_sim/autobatching.py b/torch_sim/autobatching.py
index 83c341bf..4f556ce0 100644
--- a/torch_sim/autobatching.py
+++ b/torch_sim/autobatching.py
@@ -22,7 +22,7 @@
 
 from collections.abc import Callable, Iterator
 from itertools import chain
-from typing import Any, get_args
+from typing import Any, get_args, overload
 
 import torch
 
@@ -570,6 +570,14 @@ def load_states(
 
         return self.max_memory_scaler
 
+    @overload
+    def next_batch(self, *, return_indices: bool = False) -> SimState | None: ...
+
+    @overload
+    def next_batch(
+        self, *, return_indices: bool = True
+    ) -> tuple[SimState, list[int]] | None: ...
+
     def next_batch(
         self, *, return_indices: bool = False
     ) -> SimState | tuple[SimState, list[int]] | None:
diff --git a/torch_sim/runners.py b/torch_sim/runners.py
index 14164375..5a11f86c 100644
--- a/torch_sim/runners.py
+++ b/torch_sim/runners.py
@@ -108,7 +108,7 @@ def _configure_batches_iterator(
     return batches
 
 
-def integrate(
+def integrate(  # noqa: C901
     system: StateLike,
     model: ModelInterface,
     *,
@@ -146,14 +146,19 @@ def integrate(
     """
     unit_system = UnitSystem.metal
     # create a list of temperatures
-    temps = temperature if hasattr(temperature, "__iter__") else [temperature] * n_steps
+    if hasattr(temperature, "__iter__"):
+        temps = torch.tensor(temperature, dtype=model.dtype, device=model.device)
+    else:
+        temps = torch.tensor(
+            [temperature] * n_steps, dtype=model.dtype, device=model.device
+        )
     if len(temps) != n_steps:
         raise ValueError(f"{len(temps)=:,}. It must equal n_steps = {n_steps=:,}")
 
     # initialize the state
     state: SimState = initialize_state(system, model.device, model.dtype)
     dtype, device = state.dtype, state.device
-    kTs = torch.tensor(temps, dtype=dtype, device=device) * unit_system.temperature
+    kTs = temps * unit_system.temperature
     init_fn, update_fn = integrator(
         model=model,
         kT=kTs[0],
diff --git a/torch_sim/trajectory.py b/torch_sim/trajectory.py
index fb170754..2d299772 100644
--- a/torch_sim/trajectory.py
+++ b/torch_sim/trajectory.py
@@ -30,7 +30,7 @@
 import copy
 import inspect
 import pathlib
-from collections.abc import Callable
+from collections.abc import Callable, Mapping, Sequence
 from functools import partial
 from typing import TYPE_CHECKING, Any, Literal, Self
 
@@ -44,7 +44,7 @@
 
 if TYPE_CHECKING:
     from ase import Atoms
-    from ase.io.trajectory import Trajectory
+    from ase.io.trajectory import TrajectoryReader
 
 _DATA_TYPE_MAP = {
     np.dtype("float32"): tables.Float32Atom(),
@@ -91,9 +91,18 @@ class TrajectoryReporter:
         >>> reporter.close()
     """
 
+    state_frequency: int
+    trajectory_kwargs: dict[str, Any]
+    prop_calculators: dict[int, dict[str, Callable]]
+    state_kwargs: dict[str, Any]
+    metadata: dict[str, str] | None
+    shape_warned: bool
+    trajectories: list["TorchSimTrajectory"]
+    filenames: list[str | pathlib.Path] | None
+
     def __init__(
         self,
-        filenames: str | pathlib.Path | list[str | pathlib.Path] | None,
+        filenames: str | pathlib.Path | Sequence[str | pathlib.Path] | None,
         state_frequency: int = 100,
         *,
         prop_calculators: dict[int, dict[str, Callable]] | None = None,
@@ -137,14 +146,13 @@ def __init__(
         self.trajectories = []
         if filenames is None:
             self.filenames = None
-            self.trajectories = []
         else:
             self.load_new_trajectories(filenames)
 
         self._add_model_arg_to_prop_calculators()
 
     def load_new_trajectories(
-        self, filenames: str | pathlib.Path | list[str | pathlib.Path]
+        self, filenames: str | pathlib.Path | Sequence[str | pathlib.Path]
     ) -> None:
         """Load new trajectories into the reporter.
 
@@ -159,7 +167,9 @@ def load_new_trajectories(
         """
         self.finish()
 
-        filenames = [filenames] if not isinstance(filenames, list) else filenames
+        filenames = (
+            [filenames] if isinstance(filenames, (str, pathlib.Path)) else list(filenames)
+        )
         self.filenames = [pathlib.Path(filename) for filename in filenames]
         if len(set(self.filenames)) != len(self.filenames):
             raise ValueError("All filenames must be unique.")
@@ -455,7 +465,7 @@ def _initialize_type_map(
 
     def write_arrays(
         self,
-        data: dict[str, np.ndarray | torch.Tensor],
+        data: "Mapping[str, np.ndarray | np.generic | torch.Tensor]",
         steps: int | list[int],
     ) -> None:
         """Write arrays to the trajectory file.
@@ -469,8 +479,8 @@ def write_arrays(
         file and that the steps are monotonically increasing.
 
         Args:
-            data (dict[str, np.ndarray | torch.Tensor]): Map of array names to numpy
-                arrays or torch tensors with shapes [n_frames, ...]
+            data (Mapping[str, np.ndarray | np.generic | torch.Tensor]): Map of array
+                names to numpy arrays or torch tensors with shapes [n_frames, ...]
             steps (int | list[int]): Step number(s) for the frame(s) being written.
                 If steps is an integer, arrays will be treated as single frame data.
 
@@ -485,9 +495,12 @@ def write_arrays(
             pad_first_dim = False
 
         for name, array in data.items():
-            # TODO: coerce dtypes to numpy
+            # Normalize to numpy arrays
             if isinstance(array, torch.Tensor):
                 array = array.cpu().detach().numpy()
+            elif not isinstance(array, np.ndarray):
+                # Convert numpy scalar (np.generic) or Python scalar to ndarray
+                array = np.array(array)
 
             if pad_first_dim:
                 # pad 1st dim of array with 1
@@ -715,10 +728,8 @@ def write_state(  # noqa: C901
             steps = [steps]
 
         if isinstance(system_index, int):
-            system_index = [system_index]
             sub_states = [state[system_index] for state in state]
         elif system_index is None and torch.unique(state[0].system_idx) == 0:
-            system_index = 0
             sub_states = state
         else:
             raise ValueError(
@@ -772,7 +783,7 @@ def write_state(  # noqa: C901
         # Write all arrays to file
         self.write_arrays(data, steps)
 
-    def _get_state_arrays(self, frame: int) -> dict[str, torch.Tensor]:
+    def _get_state_arrays(self, frame: int) -> dict[str, np.ndarray]:
         """Get all available state tensors for a given frame.
 
         Retrieves all state-related arrays (positions, cell, masses, etc.) for a
@@ -782,7 +793,7 @@ def _get_state_arrays(self, frame: int) -> dict[str, torch.Tensor]:
             frame (int): Frame index to retrieve (-1 for last frame)
 
         Returns:
-            dict[str, torch.Tensor]: Map of tensor names to their values
+            dict[str, np.ndarray]: Map of array names to their values
 
         Raises:
             ValueError: If required arrays are missing from trajectory or frame is
@@ -916,7 +927,7 @@ def get_state(
             positions=torch.tensor(arrays["positions"], device=device, dtype=dtype),
             masses=torch.tensor(arrays.get("masses", None), device=device, dtype=dtype),
             cell=torch.tensor(arrays["cell"], device=device, dtype=dtype),
-            pbc=arrays.get("pbc", True),
+            pbc=bool(arrays.get("pbc", True)),
             atomic_numbers=torch.tensor(
                 arrays["atomic_numbers"], device=device, dtype=torch.int
             ),
@@ -970,7 +981,7 @@ def __len__(self) -> int:
         """
         return self._file.root.data.positions.shape[0]
 
-    def write_ase_trajectory(self, filename: str | pathlib.Path) -> "Trajectory":
+    def write_ase_trajectory(self, filename: str | pathlib.Path) -> "TrajectoryReader":
         """Convert trajectory to ASE Trajectory format.
 
         Writes the entire trajectory to a new file in ASE format for compatibility
@@ -980,7 +991,7 @@ def write_ase_trajectory(self, filename: str | pathlib.Path) -> "Trajectory":
             filename (str | pathlib.Path): Path to the output ASE trajectory file
 
         Returns:
-            ase.io.trajectory.Trajectory: ASE trajectory object
+            ase.io.trajectory.TrajectoryReader: ASE trajectory object
 
         Raises:
             ImportError: If ASE is not installed