fix a bug in TranslationalSymmetry.act and add a test case for it

kwant/lattice.py

@@ -340,9 +340,19 @@ class TranslationalSymmetry(builder.Symmetry):
             raise ValueError(msg.format(self.num_directions, element))
         if b is None:
             return builder.Site(a.group, a.tag + delta, True)
-        else:
+        elif b.group is a.group:
             return builder.Site(a.group, a.tag + delta, True), \
                 builder.Site(b.group, b.tag + delta, True)
+        else:
+            m_part = self._get_site_group_data(b.group)[0]
+            try:
+                delta2 = ta.dot(m_part, element)
+            except ValueError:
+                msg = 'Expecting a {0}-tuple group element, ' + \
+                      'but got `{1}` instead.'
+                raise ValueError(msg.format(self.num_directions, element))
+            return builder.Site(a.group, a.tag + delta, True), \
+                builder.Site(b.group, b.tag + delta2, True)
 
     def to_fd(self, a, b=None):
         return self.act(-self.which(a), a, b)

kwant/tests/test_lattice.py

@@ -57,7 +57,6 @@ def test_translational_symmetry():
     assert_raises(ValueError, sym.add_site_group, g2)
 
     # Test lattices with dimension smaller than dimension of space.
-
     g2in3 = lattice.make_lattice([[4, 4, 0], [4, -4, 0]])
     sym = ts((8, 0, 0))
     sym.add_site_group(g2in3)
@@ -95,6 +94,12 @@ def test_translational_symmetry():
                 assert_equal(sym.to_fd(site2, shifted(site2, hop)),
                              (site, shifted(site, hop)))
 
+    # Test act for hoppings belonging to different lattices.
+    g2p = lattice.make_lattice(2 * np.identity(2))
+    sym = ts(*(2 * np.identity(2)))
+    assert sym.act((1, 1), g2(0, 0), g2p(0, 0)) == (g2(2, 2), g2p(1, 1))
+    assert sym.act((1, 1), g2p(0, 0), g2(0, 0)) == (g2p(1, 1), g2(2, 2))
+
 
 def test_translational_symmetry_reversed():
     def assert_equal_symmetry(a, b):