Fix slice index bug in orbital_rotation.rs (#513)

* Fix slice index panic in

* Add test for spinful orbital rotation bug

Author: davidomanovic

src/gates/orbital_rotation.rs

@@ -75,6 +75,9 @@ pub fn apply_givens_rotation_in_place(
     std::thread::scope(|scope| {
         for k in 0..n_threads {
             let start = k * chunk_size;
+            if start >= n_pairs {
+                break;
+            }
             let end = (start + chunk_size).min(n_pairs);
             let slice1_chunk = &slice1_slice[start..end];
             let slice2_chunk = &slice2_slice[start..end];

tests/python/gates/orbital_rotation_test.py

@@ -238,3 +238,18 @@ def test_apply_orbital_rotation_no_side_effects_special_case():
     assert ffsim.linalg.is_unitary(original_mat)
     assert ffsim.linalg.is_unitary(mat)
     np.testing.assert_allclose(mat, original_mat, atol=1e-12)
+
+
+def test_apply_orbital_rotation_large_spinful_regression():
+    """Regression test for slice index panic in orbital rotation."""
+    norb = 12
+    nelec = 6, 6
+    dim = ffsim.dim(norb, nelec)
+
+    rng = np.random.default_rng(123)
+    vec = ffsim.random.random_state_vector(dim, seed=rng)
+    mat_a = ffsim.random.random_unitary(norb, seed=rng)
+    mat_b = ffsim.random.random_unitary(norb, seed=rng)
+
+    result = ffsim.apply_orbital_rotation(vec, (mat_a, mat_b), norb, nelec)
+    np.testing.assert_allclose(np.linalg.norm(result), 1.0)