add util_test.py

tests/python/tenpy/util_test.py

@@ -0,0 +1,62 @@
+# (C) Copyright IBM 2025.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+import numpy as np
+import pytest
+import tenpy.linalg.np_conserved as npc
+from tenpy.networks.mps import MPS
+from tenpy.networks.site import SpinHalfFermionSite
+
+from ffsim.tenpy.util import bitstring_to_mps
+
+
+@pytest.mark.parametrize(
+    "bitstring, norb, product_state",
+    [
+        ((0, 0), 2, [0, 0]),
+        ((2, 0), 2, [0, 1]),
+        ((0, 2), 2, [0, 2]),
+        ((2, 2), 2, [0, 3]),
+        ((1, 0), 2, [1, 0]),
+        ((3, 0), 2, [1, 1]),
+        ((1, 2), 2, [1, 2]),
+        ((3, 2), 2, [1, 3]),
+        ((0, 1), 2, [2, 0]),
+        ((2, 1), 2, [2, 1]),
+        ((0, 3), 2, [2, 2]),
+        ((2, 3), 2, [2, 3]),
+        ((1, 1), 2, [3, 0]),
+        ((3, 1), 2, [3, 1]),
+        ((1, 3), 2, [3, 2]),
+        ((3, 3), 2, [3, 3]),
+    ],
+)
+def test_bitstring_to_mps(bitstring: tuple[int, int], norb: int, product_state: list):
+    """Test converting a bitstring to an MPS."""
+
+    # convert bitstring to MPS
+    mps = bitstring_to_mps(bitstring, norb)
+
+    # construct expected MPS
+    shfs = SpinHalfFermionSite(cons_N="N", cons_Sz="Sz")
+    expected_mps = MPS.from_product_state([shfs] * norb, product_state)
+
+    # map from ffsim to TeNPy ordering
+    if product_state[0] in [2, 3] and product_state[1] in [1, 3]:
+        minus_identity_npc = npc.Array.from_ndarray(
+            -shfs.get_op("Id").to_ndarray(),
+            [shfs.leg, shfs.leg.conj()],
+            labels=["p", "p*"],
+        )
+        expected_mps.apply_local_op(0, minus_identity_npc)
+
+    # test overlap is one
+    overlap = mps.overlap(expected_mps)
+    np.testing.assert_equal(overlap, 1)