From 3b0727cb99f7a78b40f1ce0e61069b4dd41023ec Mon Sep 17 00:00:00 2001
From: lkdvos <ldevos98@gmail.com>
Date: Wed, 23 Sep 2026 11:43:58 -0400
Subject: [PATCH 1/2] Fix SpaceMismatch in canonicalize! for partially
 decoupled Hamiltonian sites

left_canonicalize!/right_canonicalize! forced both virtual legs of the
reconstructed A-block empty in the "fully truncated" (dim(R)==0) branch,
but only the leg actually being decomposed collapses when a bond only
partially decouples. Inherit the other leg's real space instead.

Fixes #522

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 src/operators/ortho.jl     |  4 ++--
 test/hamiltonian/finite.jl | 40 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/src/operators/ortho.jl b/src/operators/ortho.jl
index de7d00200..9ea1f64a5 100644
--- a/src/operators/ortho.jl
+++ b/src/operators/ortho.jl
@@ -50,7 +50,7 @@ function left_canonicalize!(
 
         if dim(R) == 0 # fully truncated
             V = _rightunit ⊞ _rightunit
-            Q1 = typeof(WA)(undef, SumSpace{S}() ⊗ P ← P ⊗ SumSpace{S}())
+            Q1 = typeof(WA)(undef, codomain(WA) ← P ⊗ SumSpace{S}())
             Q2 = typeof(WC)(undef, P ← P ⊗ SumSpace{S}())
         else
             scale!(Q, d)
@@ -144,7 +144,7 @@ function right_canonicalize!(
         R, Q = right_orth!(tmp; alg)
         if dim(R) == 0
             V = _rightunit ⊞ _rightunit
-            Q1 = typeof(WA)(undef, SumSpace{S}() ⊗ P ← P ⊗ SumSpace{S}())
+            Q1 = typeof(WA)(undef, SumSpace{S}() ⊗ P ← domain(WA))
             Q2 = typeof(WB)(undef, SumSpace{S}() ⊗ P ← P)
         else
             scale!(Q, d)
diff --git a/test/hamiltonian/finite.jl b/test/hamiltonian/finite.jl
index 950f855fc..e3c266249 100644
--- a/test/hamiltonian/finite.jl
+++ b/test/hamiltonian/finite.jl
@@ -109,6 +109,46 @@ using TensorKit: ℙ
     end
 end
 
+@testset "Finite MPOHamiltonian canonicalization with decoupled sites" begin
+    # regression test for https://github.com/QuantumKitHub/MPSKit.jl/issues/522
+    T = ComplexF64
+    for V in (ℂ^2, U1Space(-1 => 1, 0 => 1, 1 => 1), Rep[SU₂](1 // 2 => 1))
+        O₁ = randn(T, V, V)
+        O₁ += O₁'
+        O₂ = randn(T, V^2 ← V^2)
+        O₂ += O₂'
+        E = id(storagetype(O₁), domain(O₁))
+
+        # bulk block of the middle site is empty on the left but not on the right
+        H_right = FiniteMPOHamiltonian(fill(V, 3), (1,) => O₁, (2, 3) => O₂)
+        H_right_tm = O₁ ⊗ E ⊗ E + E ⊗ O₂
+        H = MPSKit.right_canonicalize!(copy(H_right), 2)
+        @test convert(TensorMap, H) ≈ H_right_tm
+
+        # bulk block of the middle site is empty on the right but not on the left
+        H_left = FiniteMPOHamiltonian(fill(V, 3), (1, 2) => O₂, (3,) => O₁)
+        H_left_tm = O₂ ⊗ E + E ⊗ E ⊗ O₁
+        H = MPSKit.left_canonicalize!(copy(H_left), 2)
+        @test convert(TensorMap, H) ≈ H_left_tm
+
+        L = 5
+        H5 = FiniteMPOHamiltonian(fill(V, L), (1, 2) => O₂, (3,) => O₁, (4, 5) => O₂)
+        H5_tm = O₂ ⊗ E ⊗ E ⊗ E + E ⊗ E ⊗ O₁ ⊗ E ⊗ E + E ⊗ E ⊗ E ⊗ O₂
+        H = copy(H5)
+        for i in L:-1:2
+            MPSKit.right_canonicalize!(H, i)
+        end
+        @test convert(TensorMap, H) ≈ H5_tm
+        for i in 1:(L - 1)
+            MPSKit.left_canonicalize!(H, i)
+        end
+        @test convert(TensorMap, H) ≈ H5_tm
+
+        H5_trunc = changebonds(H5, SvdCut(; trunc = truncrank(16)))
+        @test convert(TensorMap, H5_trunc) ≈ H5_tm
+    end
+end
+
 @testset "Finite MPOHamiltonian repeated indices" begin
     X = randn(ComplexF64, ℂ^2, ℂ^2)
     X += X'

From f7e055efdf22bda7cfe7b1e9b7ef9475b727794d Mon Sep 17 00:00:00 2001
From: lkdvos <ldevos98@gmail.com>
Date: Thu, 24 Sep 2026 14:15:07 -0400
Subject: [PATCH 2/2] Bump BlockTensorKit compat to 0.3.19

BlockTensorKit 0.3.17/0.3.18 silently returned zero from sparse
block-tensor contractions with 3+ legs, corrupting
convert(TensorMap, ::FiniteMPOHamiltonian) and related densification
paths for essentially any Hamiltonian. 0.3.19 fixes this upstream;
tests (operators/mpohamiltonian, algorithms/changebonds) pass cleanly
against it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 Project.toml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Project.toml b/Project.toml
index f6c5866f0..740f72d0f 100644
--- a/Project.toml
+++ b/Project.toml
@@ -37,7 +37,7 @@ MPSKitAdaptExt = "Adapt"
 [compat]
 Accessors = "0.1"
 Adapt = "4"
-BlockTensorKit = "0.3.16"
+BlockTensorKit = "0.3.19"
 Compat = "3.47, 4.10"
 DocStringExtensions = "0.9.3"
 HalfIntegers = "1.6.0"