Fix test for Kramers pairs

Author: greschd

tests/test_z2_invariant.py

@@ -35,7 +35,14 @@ def test_trivial(num_lines, num_wcc, patch_surface_data):
     """
     wcc = [np.linspace(0, 1, num_wcc) for j in range(num_lines)]
     data = SurfaceData(wcc)
-    assert z2pack.invariant.z2(data) == 0
+    assert z2pack.invariant.z2(data, check_kramers_pairs=False) == 0
+
+
+def test_no_kramers_pairs(N, M, patch_surface_data):  # pylint: disable=invalid-name
+    wcc = [np.linspace(0, 1, 2 * M + 1) for j in range(N + 1)]
+    data = SurfaceData(wcc)
+    with pytest.raises(ValueError):
+        z2pack.invariant.z2(data)
 
 
 def test_linear(num_lines_nonzero, x, patch_surface_data):

z2pack/_utils.py

@@ -69,3 +69,16 @@ def _pol_step(pol_list):
     for pol_left, pol_right in zip(pol_list[:-1], pol_list[1:]):
         res.append(min((pol_right - pol_left + o for o in offset), key=abs))
     return res
+
+
+def _check_kramers_pairs(wcc, tol=1e-3):
+    return _get_degenerate_dist(wcc) < tol
+
+
+def _get_degenerate_dist(wcc):
+    """
+    Get the maximum distance between pairs of approximately degenerate WCC.
+    """
+    if len(wcc) % 2 != 0:
+        raise ValueError('The number of WCC must be even!')
+    return _get_max_move(wcc[::2], wcc[1::2])

z2pack/invariant.py

@@ -6,7 +6,7 @@
 
 from fsc.export import export
 
-from ._utils import _pol_step, _sgng
+from ._utils import _pol_step, _sgng, _check_kramers_pairs
 
 
 @export
@@ -28,13 +28,16 @@ def chern(surface_result):
 
 
 @export
-def z2(surface_result):  # pylint: disable=invalid-name
+def z2(surface_result, check_kramers_pairs=True):  # pylint: disable=invalid-name
     r"""
     Computes the :math:`\mathbb{Z}_2` invariant corresponding to a given surface result.
 
     :param surface_result: Result of a WCC calculation on a Surface.
     :type surface_result: :class:`.SurfaceResult` or :class:`.SurfaceData`
 
+    :param check_kramers_pairs: Check whether the WCC at the edge of the surface come in degenerate pairs. This is a requirement for having a well-defined Z2 invariant.
+    :type check_kramers_pairs: bool
+
     Example code:
 
     .. code :: python
@@ -43,6 +46,13 @@ def z2(surface_result):  # pylint: disable=invalid-name
         print(z2pack.invariant.z2(result)) # Prints the Z2 invariant
     """
     wcc = surface_result.wcc
+    if check_kramers_pairs and len(wcc) > 0:  # pylint: disable=len-as-condition
+        if not _check_kramers_pairs(list(wcc[0])) and _check_kramers_pairs(
+            list(wcc[-1])
+        ):
+            raise ValueError(
+                'The given WCC are not degenerate Kramers pairs at the edges of the surface.'
+            )
     gap = surface_result.gap_pos
     inv = 1
     for gap_left, gap_right, wcc_right in zip(gap, gap[1:], wcc[1:]):