diff --git a/examples/vumps/vumps_2d_heisenberg.jl b/examples/vumps/vumps_2d_heisenberg.jl
index 968606d..35118d1 100644
--- a/examples/vumps/vumps_2d_heisenberg.jl
+++ b/examples/vumps/vumps_2d_heisenberg.jl
@@ -34,9 +34,9 @@ function ITensorInfiniteMPS.unit_cell_terms(::Model"heisenberg2D"; width)
   opsum = OpSum()
   for i in 1:width
     # Vertical
-    opsum += -0.5, "S+", i, "S-", mod(i + 1, width)
-    opsum += -0.5, "S-", i, "S+", mod(i + 1, width)
-    opsum += "Sz", i, "Sz", mod(i + 1, width)
+    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
     # Horizontal
     opsum += -0.5, "S+", i, "S-", i + width
     opsum += -0.5, "S-", i, "S+", i + width
diff --git a/src/models/models.jl b/src/models/models.jl
index 206defd..ac0b3a2 100644
--- a/src/models/models.jl
+++ b/src/models/models.jl
@@ -245,6 +245,12 @@ function infinite_terms(opsum::OpSum; kwargs...)
   # stores all terms starting on site `i`.
   opsum_cell_dict = groupreduce(minimum ∘ ITensors.sites, +, opsum)
   nsites = maximum(keys(opsum_cell_dict))
+  # check that we don't have terms we will ignore
+  dropped = filter(x -> x <= 0, keys(opsum_cell_dict))
+  if length(dropped) > 0
+    @warn "The input unit cell terms include terms that are being ignored on sites: $([d for d in dropped])"
+  end
+
   # Assumes each site in the unit cell has a term
   for j in 1:nsites
     if !haskey(opsum_cell_dict, j)