From a763414d5fb7b9f0ac2c4afb89ec477683719d12 Mon Sep 17 00:00:00 2001
From: Benedikt Kloss <bene.kloss@gmail.com>
Date: Wed, 6 Mar 2024 15:55:08 -0500
Subject: [PATCH 1/3] Make internal vertices without siteindices compare equal
 in `hascommoninds`.

---
 src/abstractitensornetwork.jl | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/abstractitensornetwork.jl b/src/abstractitensornetwork.jl
index 6905694b..8e7b40c0 100644
--- a/src/abstractitensornetwork.jl
+++ b/src/abstractitensornetwork.jl
@@ -798,7 +798,11 @@ function hascommoninds(
   ::typeof(siteinds), A::AbstractITensorNetwork{V}, B::AbstractITensorNetwork{V}
 ) where {V}
   for v in vertices(A)
-    !hascommoninds(siteinds(A, v), siteinds(B, v)) && return false
+    if isempty(siteinds(A, v))
+      !isempty(siteinds(B, v)) && return false
+    else
+      !hascommoninds(siteinds(A, v), siteinds(B, v)) && return false
+    end
   end
   return true
 end
@@ -815,9 +819,13 @@ function check_hascommoninds(
     )
   end
   for v in vertices(A)
-    !hascommoninds(siteinds(A, v), siteinds(B, v)) && error(
-      "$(typeof(A)) A and $(typeof(B)) B must share site indices. On vertex $v, A has site indices $(siteinds(A, v)) while B has site indices $(siteinds(B, v)).",
-    )
+    if isempty(siteinds(A, v))
+      !(isempty(siteinds(B, v))) && return false
+    else
+      !hascommoninds(siteinds(A, v), siteinds(B, v)) && error(
+        "$(typeof(A)) A and $(typeof(B)) B must share site indices. On vertex $v, A has site indices $(siteinds(A, v)) while B has site indices $(siteinds(B, v)).",
+      )
+    end
   end
   return nothing
 end

From e3d4793ddd2f99ceb1bfdce4960cd73b2ceb18d5 Mon Sep 17 00:00:00 2001
From: Benedikt Kloss <bene.kloss@gmail.com>
Date: Wed, 6 Mar 2024 18:48:11 -0500
Subject: [PATCH 2/3] Add test.

---
 test/test_opsum_to_ttn.jl | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/test/test_opsum_to_ttn.jl b/test/test_opsum_to_ttn.jl
index 810d7f03..efdd2bd4 100644
--- a/test/test_opsum_to_ttn.jl
+++ b/test/test_opsum_to_ttn.jl
@@ -247,7 +247,6 @@ end
         Tttno = contract(Hsvd)
       end
       @test Tttno ≈ Tmpo rtol = 1e-6
-
       Hsvd_lr = TTN(
         Hlr, is_missing_site; root_vertex=root_vertex, algorithm="svd", cutoff=1e-10
       )
@@ -257,6 +256,10 @@ end
         Tmpo_lr = contract(Hsvd_lr)
       end
       @test Tttno_lr ≈ Tmpo_lr rtol = 1e-6
+      # ToDo: move to a more appropriate file, since this is not related to OpSum to TTN conversion
+      # test whether Hamiltonian contracts with itself (used to error due to missing siteindices)
+      @test isa(inner(Hsvd, Hsvd), Number)
     end
   end
 end
+return nothing

From 576bed31501e4af731d943a894845cba5146afab Mon Sep 17 00:00:00 2001
From: Benedikt Kloss <bene.kloss@gmail.com>
Date: Wed, 6 Mar 2024 18:48:32 -0500
Subject: [PATCH 3/3] Add error message.

---
 src/abstractitensornetwork.jl | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/abstractitensornetwork.jl b/src/abstractitensornetwork.jl
index 8e7b40c0..10fc6a1d 100644
--- a/src/abstractitensornetwork.jl
+++ b/src/abstractitensornetwork.jl
@@ -820,7 +820,9 @@ function check_hascommoninds(
   end
   for v in vertices(A)
     if isempty(siteinds(A, v))
-      !(isempty(siteinds(B, v))) && return false
+      !(isempty(siteinds(B, v))) && error(
+        "$(typeof(A)) A and $(typeof(B)) B must share site indices. On vertex $v, A has no site indices while B has site indices $(siteinds(B, v)).",
+      )
     else
       !hascommoninds(siteinds(A, v), siteinds(B, v)) && error(
         "$(typeof(A)) A and $(typeof(B)) B must share site indices. On vertex $v, A has site indices $(siteinds(A, v)) while B has site indices $(siteinds(B, v)).",