diff --git a/atompack-py/tests/test_atom_molecule.py b/atompack-py/tests/test_atom_molecule.py
index 0ff034c..9b12293 100644
--- a/atompack-py/tests/test_atom_molecule.py
+++ b/atompack-py/tests/test_atom_molecule.py
@@ -262,3 +262,38 @@ def test_molecule_getitem_validation() -> None:
         _ = mol["does_not_exist"]
     with pytest.raises(TypeError, match=r"integers or strings"):
         _ = mol[1.5]
+
+
+def test_from_arrays_rejects_wrong_dtype_positions() -> None:
+    # positions must be float32; passing float64 should be rejected cleanly,
+    # not silently truncated or panic across the FFI boundary.
+    positions = np.zeros((2, 3), dtype=np.float64)  # wrong dtype
+    atomic_numbers = np.array([6, 8], dtype=np.uint8)
+    with pytest.raises((TypeError, ValueError)):
+        atompack.Molecule.from_arrays(positions, atomic_numbers)
+
+
+def test_from_arrays_rejects_wrong_dtype_atomic_numbers() -> None:
+    # atomic_numbers must be uint8; passing int64 should be rejected cleanly.
+    positions = np.zeros((2, 3), dtype=np.float32)
+    atomic_numbers = np.array([6, 8], dtype=np.int64)  # wrong dtype
+    with pytest.raises((TypeError, ValueError)):
+        atompack.Molecule.from_arrays(positions, atomic_numbers)
+
+
+def test_set_property_python_bool_pins_current_behavior() -> None:
+    # Pin the (perhaps surprising) current behavior: Python bool is a subclass
+    # of int, and PyO3 extracts it as i64 before f64. So set_property(True)
+    # stores Int(1), not Bool. This test locks in the current contract; if
+    # anyone wants real bool storage they should add a TYPE_BOOL tag and
+    # reorder extract attempts.
+    mol = _make_molecule()
+    mol.set_property("flag_true", True)
+    mol.set_property("flag_false", False)
+
+    flag_true = mol.get_property("flag_true")
+    flag_false = mol.get_property("flag_false")
+    assert isinstance(flag_true, int) and not isinstance(flag_true, bool)
+    assert flag_true == 1
+    assert isinstance(flag_false, int) and not isinstance(flag_false, bool)
+    assert flag_false == 0
diff --git a/atompack-py/tests/test_database.py b/atompack-py/tests/test_database.py
index 7fbb075..65cafb5 100644
--- a/atompack-py/tests/test_database.py
+++ b/atompack-py/tests/test_database.py
@@ -249,7 +249,7 @@ def test_database_add_arrays_batch_roundtrip_with_custom_properties(tmp_path: Pa
 
 
 @pytest.mark.parametrize("mmap", [False, True])
-@pytest.mark.parametrize("compression", ["none", "zstd"])
+@pytest.mark.parametrize("compression", ["none", "lz4", "zstd"])
 def test_database_single_item_reads_are_view_compatible(
     tmp_path: Path,
     mmap: bool,
@@ -283,7 +283,7 @@ def test_database_single_item_reads_are_view_compatible(
 
 
 @pytest.mark.parametrize("mmap", [False, True])
-@pytest.mark.parametrize("compression", ["none", "zstd"])
+@pytest.mark.parametrize("compression", ["none", "lz4", "zstd"])
 def test_database_custom_array_properties_roundtrip_all_numeric_tags(
     tmp_path: Path,
     mmap: bool,
@@ -349,7 +349,7 @@ def test_database_custom_array_properties_roundtrip_all_numeric_tags(
 
 
 @pytest.mark.parametrize("mmap", [False, True])
-@pytest.mark.parametrize("compression", ["none", "zstd"])
+@pytest.mark.parametrize("compression", ["none", "lz4", "zstd"])
 def test_database_single_item_mutation_is_copy_on_write(
     tmp_path: Path,
     mmap: bool,
@@ -392,7 +392,7 @@ def test_database_single_item_mutation_is_copy_on_write(
 
 
 @pytest.mark.parametrize("mmap", [False, True])
-@pytest.mark.parametrize("compression", ["none", "zstd"])
+@pytest.mark.parametrize("compression", ["none", "lz4", "zstd"])
 def test_database_single_item_pickle_materializes_owned_roundtrip(
     tmp_path: Path,
     mmap: bool,
@@ -577,3 +577,57 @@ def test_get_molecules_flat_empty(tmp_path: Path) -> None:
     assert batch["n_atoms"].shape == (0,)
     assert batch["positions"].shape == (0, 3)
     assert batch["atomic_numbers"].shape == (0,)
+
+
+def test_database_open_mmap_populate(tmp_path: Path) -> None:
+    # Smoke test for the documented populate=True path. On Linux this
+    # prefaults mapped pages via memmap2's PopulateRead advise; on macOS the
+    # advise is best-effort and silently ignored, so the assertion here is
+    # only that the open path succeeds and returns correct data — not that
+    # pages are actually faulted in. Pre-faulting performance is exercised
+    # in benchmarks/, not unit tests.
+    path = tmp_path / "populate.atp"
+    db = atompack.Database(str(path))
+    db.add_molecule(_make_molecule(-3.0))
+    db.flush()
+
+    db_r = atompack.Database.open(str(path), mmap=True, populate=True)
+    assert len(db_r) == 1
+    assert db_r[0].energy == pytest.approx(-3.0)
+
+
+def test_database_negative_indexing_raises_overflow_error(tmp_path: Path) -> None:
+    # Database does not support negative indexing today. PyO3 extracts the
+    # index argument as `usize`, so a negative integer raises OverflowError
+    # at the FFI boundary. If wraparound semantics are ever added, this
+    # test will fail loudly so the intent is explicit.
+    path = tmp_path / "negidx.atp"
+    db = atompack.Database(str(path))
+    db.add_molecule(_make_molecule(-1.0))
+    db.flush()
+
+    db_r = atompack.Database.open(str(path))
+    with pytest.raises(OverflowError, match=r"negative"):
+        _ = db_r[-1]
+
+
+def test_database_empty_molecule_roundtrip(tmp_path: Path) -> None:
+    # n_atoms == 0 is the SOA parser edge case; positions/atomic_numbers slices
+    # become zero-length and most code paths must still work.
+    path = tmp_path / "empty_mol.atp"
+    mol = atompack.Molecule.from_arrays(
+        np.zeros((0, 3), dtype=np.float32),
+        np.zeros((0,), dtype=np.uint8),
+        energy=0.0,
+    )
+
+    db = atompack.Database(str(path))
+    db.add_molecule(mol)
+    db.flush()
+
+    db_r = atompack.Database.open(str(path))
+    read = db_r[0]
+    assert len(read) == 0
+    assert read.positions.shape == (0, 3)
+    assert read.atomic_numbers.shape == (0,)
+    assert read.energy == pytest.approx(0.0)
diff --git a/atompack-py/tests/test_from_ase.py b/atompack-py/tests/test_from_ase.py
index 542cdd4..2a2a34a 100644
--- a/atompack-py/tests/test_from_ase.py
+++ b/atompack-py/tests/test_from_ase.py
@@ -502,3 +502,29 @@ def test_to_ase_batch_none_calc_mode_preserves_results(tmp_path) -> None:
     np.testing.assert_allclose(atoms_batch[1].info["stress"], stress[1])
     np.testing.assert_allclose(atoms_batch[1].arrays["forces"], forces[1])
     np.testing.assert_allclose(atoms_batch[1].arrays["charges"], charges[1])
+
+
+def test_from_ase_expands_voigt6_stress_to_3x3() -> None:
+    # ASE's get_stress(voigt=True) returns a (6,) Voigt-form array; the bridge
+    # must expand it to a (3,3) symmetric tensor before storing.
+    voigt = np.array([1.1, 2.2, 3.3, 4.4, 5.5, 6.6], dtype=np.float64)
+    atoms = FakeASEAtoms(
+        positions=np.array([[0.0, 0.0, 0.0], [1.0, 0.0, 0.0]], dtype=np.float64),
+        atomic_numbers=np.array([6, 8], dtype=np.int64),
+        info={"ase_stress": voigt},
+    )
+
+    mol = atompack.from_ase(atoms)
+    assert mol.stress is not None
+    assert mol.stress.shape == (3, 3)
+    # Voigt order is (xx, yy, zz, yz, xz, xy); expanded matrix is symmetric.
+    expected = np.array(
+        [
+            [voigt[0], voigt[5], voigt[4]],
+            [voigt[5], voigt[1], voigt[3]],
+            [voigt[4], voigt[3], voigt[2]],
+        ],
+        dtype=np.float64,
+    )
+    np.testing.assert_allclose(mol.stress, expected)
+    np.testing.assert_allclose(mol.stress, mol.stress.T)  # symmetry sanity