Hubbard model periodic fix (#30)

* added hubbard model test that fails

* fixed bottom_neighbor for periodic boundary conditions

* added line break to conform with line width limit

* fixed bug with right/left periodic conditions

src/openfermion/utils/_hubbard.py

@@ -129,7 +129,7 @@ def fermi_hubbard(x_dimension, y_dimension, tunneling, coulomb,
         # Account for periodic boundaries.
         if periodic:
             if (x_dimension > 2) and ((site + 1) % x_dimension == 0):
-                right_neighbor -= (x_dimension - 1)
+                right_neighbor -= x_dimension
             if (y_dimension > 2) and (site + x_dimension + 1 > n_sites):
                 bottom_neighbor -= x_dimension * y_dimension
 

src/openfermion/utils/_hubbard_test.py

@@ -102,7 +102,6 @@ def test_two_by_two_spinless_periodic_rudimentary(self):
             self.chemical_potential, self.magnetic_field,
             periodic=True, spinless=True)
 
-
     def test_two_by_three_spinless_periodic_rudimentary(self):
         hubbard_model = fermi_hubbard(
             2, 3, self.tunneling, self.coulomb,
@@ -111,3 +110,12 @@ def test_two_by_three_spinless_periodic_rudimentary(self):
         # Check up top/bottom hopping terms.
         self.assertAlmostEqual(hubbard_model.terms[((4, 1), (0, 0))],
                                -self.tunneling)
+
+    def test_three_by_two_spinless_periodic_rudimentary(self):
+        hubbard_model = fermi_hubbard(
+            3, 2, self.tunneling, self.coulomb,
+            self.chemical_potential, self.magnetic_field,
+            periodic=True, spinless=True)
+        # Check up top/bottom hopping terms.
+        self.assertAlmostEqual(hubbard_model.terms[((2, 1), (0, 0))],
+                               -self.tunneling)