Fix yperiodic term

Author: ryanlevy

examples/vumps/vumps_2d_heisenberg.jl

@@ -35,19 +35,16 @@ function ITensorInfiniteMPS.unit_cell_terms(::Model"heisenberg2D"; width, yperio
   opsum = OpSum()
   for i in 1:width
     # Vertical
-    opsum -= 0.5, "S+", i, "S-", i + 1
-    opsum -= 0.5, "S-", i, "S+", i + 1
-    opsum += "Sz", i, "Sz", i + 1
+    if i < width || yperiodic
+      opsum -= 0.5, "S+", i, "S-", mod(i, width) + 1
+      opsum -= 0.5, "S-", i, "S+", mod(i, width) + 1
+      opsum += "Sz", i, "Sz", mod(i, width) + 1
+    end
     # Horizontal
     opsum -= 0.5, "S+", i, "S-", i + width
     opsum -= 0.5, "S-", i, "S+", i + width
     opsum += "Sz", i, "Sz", i + width
   end
-  if yperiodic
-    opsum -= 0.5, "S+", 1, "S-", width
-    opsum -= 0.5, "S-", 1, "S+", width
-    opsum += "Sz", 1, "Sz", width
-  end
   return opsum
 end
 model = Model("heisenberg2D")