diff --git a/src/IntrTree/addIntr.jl b/src/IntrTree/addIntr.jl
index e04f369..e164c83 100644
--- a/src/IntrTree/addIntr.jl
+++ b/src/IntrTree/addIntr.jl
@@ -98,6 +98,37 @@ function _addZ!(OR::LocalOperator{2, 2}, Z::AbstractTensorMap)
      return OR
 end
 
+# swap two operators to deal with horizontal bond
+_swap(A::LocalOperator{1, 1}, B::LocalOperator{1, 1}) = B, A
+_swap(A::LocalOperator{1, 1}, B::LocalOperator{1, 2}) = B, A
+_swap(A::LocalOperator{2, 1}, B::LocalOperator{1, 1}) = B, A
+function _swap(A::LocalOperator{1, 2}, B::LocalOperator{2, 1})
+     return _swapOp(B), _swapOp(A)
+end
+function _swap(A::LocalOperator{1, 2}, B::LocalOperator{2, 2})
+     #  |      |          |      |
+     #  A--  --B--va -->  B--  --A--va
+     #  |      |          |      |
+
+     @tensor AB[d e; a b f] := A.A[a b c] * B.A[c d e f]     
+     # QR 
+     TA, TB = leftorth(AB)
+     
+     return LocalOperator(permute(TA, (1,), (2, 3)), B.name, B.si, B.strength), LocalOperator(permute(TB, (1, 2), (3, 4)), A.name, A.si, A.strength)
+end
+function _swap(A::LocalOperator{2, 2}, B::LocalOperator{2,1})
+     #     |     |         |     |
+     # va--A-- --B --> va--B-- --A 
+     #     |     |         |     |
+
+     @tensor AB[a e f; b c] := A.A[a b c d] * B.A[d e f]
+     # QR
+     TA, TB = rightorth(AB)
+
+     return LocalOperator(permute(TA, (1, 2), (3, 4)), B.name, B.si, B.strength), LocalOperator(permute(TB, (1, 2), (3,)), A.name, A.si, A.strength)
+end
+
+
 
 
 
diff --git a/src/IntrTree/addIntr4.jl b/src/IntrTree/addIntr4.jl
index 5a02837..5195733 100644
--- a/src/IntrTree/addIntr4.jl
+++ b/src/IntrTree/addIntr4.jl
@@ -38,20 +38,25 @@ function addIntr4!(Root::InteractionTreeNode, O::NTuple{4,AbstractTensorMap}, si
      (A, B, C, D) = map(1:4) do i
           LocalOperator(O[i], name[i], si[i])
      end
-     _addtag!(A, B, C, D)
 
      # deal with the permutation 1<->2 and 3<->4
      if si[1] > si[2]
-          A, B = _leftOp(B), _rightOp(A)
+          A, B = _swap(A, B)
           si = (si[2], si[1], si[3], si[4])
           !isnothing(Z) && (strength *= -1)
      end
      if si[3] > si[4]
-          C, D = _leftOp(D), _rightOp(C)
+          C, D = _swap(C, D)
           si = (si[1], si[2], si[4], si[3])
           !isnothing(Z) && (strength *= -1)
      end
 
+     _addtag!(A, B, C, D)
+
+     # ============ reduce to 2-site term ===========
+     if A.si == B.si && C.si == D.si
+          return addIntr2!(Root, A * B, C * D, strength, nothing; value = value)
+     end
      if A.si == C.si && B.si == D.si
           #          D
           #  C Z Z Z Z              -BD