catch problem with no bands open

kwant_spectrum.py

@@ -804,7 +804,11 @@ class BandSketching:
         assert _is_type(band, 'integer')
 
         energy = self.__call__(k, band)
-        modes = self._modes(energy=energy)[0]
+        try:  # kwant fails if energy is exactly at the bandgap
+            modes = self._modes(energy=energy)[0]
+        except ValueError:
+            return -1
+
         momenta = modes.momenta
         velocities = modes.velocities
 

test_kwant_spectrum.py

@@ -139,7 +139,7 @@ def test_gap_detection_tolerance():
 def test_scale_invariance():
     def make_lead(scale=1):
         syst = kwant.Builder(kwant.TranslationalSymmetry((-1, 0)))
-        lat = kwant.lattice.square()
+        lat = kwant.lattice.square(norbs=1)
         syst[lat(0, 0)] = 1*scale
         syst[lat(0, 1)] = 8*scale
         syst[lat(1, 0), lat(0, 0)] = -1*scale
@@ -160,7 +160,7 @@ def test_band_analysis_methods():
     # a simple testcase with 3 crossing bands
     def make_lead():
         syst = kwant.Builder(kwant.TranslationalSymmetry((-2, 0)))
-        lat = kwant.lattice.square(1)
+        lat = kwant.lattice.square(1, norbs=1)
         syst[lat(0, 0)] = 0
         syst[lat(0, 1)] = 0
         syst[lat(1, 0)] = 0
@@ -171,7 +171,7 @@ def test_band_analysis_methods():
 
     # a simple testcase with 3 non-crossing bands
     def make_simple_lead(W=3):
-        lat = kwant.lattice.square(1)
+        lat = kwant.lattice.square(1, norbs=1)
         sym = kwant.TranslationalSymmetry((-1, 0))
         H = kwant.Builder(sym)
         H[(lat(0, y) for y in range(W))] = 0