From 989c1d635a6b9b893c5ba867ef6fe31834e1bc5b Mon Sep 17 00:00:00 2001
From: herviou <loic.herviou@gmail.com>
Date: Fri, 17 Apr 2026 10:53:53 +0200
Subject: [PATCH 1/5] Fixing changebonds, some inconsistency remaining

---
 src/algorithms/changebonds/changebonds.jl   |  5 ++++-
 src/algorithms/changebonds/optimalexpand.jl | 11 +++++++++++
 src/algorithms/changebonds/randexpand.jl    |  6 ++++++
 src/algorithms/changebonds/svdcut.jl        |  5 +++--
 src/states/finitemps.jl                     |  9 +++++++++
 5 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/src/algorithms/changebonds/changebonds.jl b/src/algorithms/changebonds/changebonds.jl
index 700b5ab0b..3cff7c4b1 100644
--- a/src/algorithms/changebonds/changebonds.jl
+++ b/src/algorithms/changebonds/changebonds.jl
@@ -9,15 +9,18 @@ See also: [`SvdCut`](@ref), [`RandExpand`](@ref), [`VUMPSSvdCut`](@ref), [`Optim
 function changebonds end
 function changebonds! end
 
+
+##= This is never useful
 # write in terms of MultilineMPS
 function changebonds(
-        ψ::InfiniteMPS, operator::InfiniteMPO, alg, envs = environments(ψ, operator)
+        ψ::InfiniteMPS, operator::InfiniteMPO, alg::OptimalExpand, envs = environments(ψ, operator)
     )
     ψ′, envs′ = changebonds(
         convert(MultilineMPS, ψ), convert(MultilineMPO, operator), alg, Multiline([envs])
     )
     return convert(InfiniteMPS, ψ′), envs
 end
+# =#
 
 _expand(ψ, AL′, AR′) = _expand!(copy(ψ), AL′, AR′)
 function _expand!(ψ::InfiniteMPS, AL′::PeriodicVector, AR′::PeriodicVector)
diff --git a/src/algorithms/changebonds/optimalexpand.jl b/src/algorithms/changebonds/optimalexpand.jl
index c33c358c4..59fd002ca 100644
--- a/src/algorithms/changebonds/optimalexpand.jl
+++ b/src/algorithms/changebonds/optimalexpand.jl
@@ -17,6 +17,17 @@ $(TYPEDFIELDS)
     trscheme::TruncationStrategy
 end
 
+##= This is only useful for OptimalExpand
+function changebonds(
+        ψ::InfiniteMPS, operator::InfiniteMPO, alg::OptimalExpand, envs = environments(ψ, operator)
+    )
+    ψ′, envs′ = changebonds(
+        convert(MultilineMPS, ψ), convert(MultilineMPO, operator), alg, Multiline([envs])
+    )
+    return convert(InfiniteMPS, ψ′), envs #This does not sound safe, it relies on the onsite modification of the environments, and the compatibility of the conversion
+end
+# =#
+
 function changebonds(
         ψ::InfiniteMPS, H::InfiniteMPOHamiltonian, alg::OptimalExpand,
         envs = environments(ψ, H)
diff --git a/src/algorithms/changebonds/randexpand.jl b/src/algorithms/changebonds/randexpand.jl
index 80003ead2..e67a05509 100644
--- a/src/algorithms/changebonds/randexpand.jl
+++ b/src/algorithms/changebonds/randexpand.jl
@@ -50,6 +50,12 @@ end
 changebonds(ψ::AbstractMPS, alg::RandExpand) = changebonds!(copy(ψ), alg)
 changebonds(ψ::MultilineMPS, alg::RandExpand) = changebonds!(copy(ψ), alg)
 
+function changebonds(ψ, H, alg::RandExpand, envs)
+  newψ = changebonds(ψ, alg)    
+  return newψ, environments(newψ, H)
+end
+
+
 function changebonds!(ψ::AbstractFiniteMPS, alg::RandExpand)
     for i in 1:(length(ψ) - 1)
         AC2 = randomize!(_transpose_front(ψ.AC[i]) * _transpose_tail(ψ.AR[i + 1]))
diff --git a/src/algorithms/changebonds/svdcut.jl b/src/algorithms/changebonds/svdcut.jl
index d5d4ea48a..2b674d5be 100644
--- a/src/algorithms/changebonds/svdcut.jl
+++ b/src/algorithms/changebonds/svdcut.jl
@@ -108,8 +108,9 @@ function changebonds(ψ::InfiniteMPS, alg::SvdCut)
     return normalize!(ψ)
 end
 
-function changebonds(ψ, H, alg::SvdCut, envs = environments(ψ, H))
-    return changebonds(ψ, alg), envs
+function changebonds(ψ, H, alg::SvdCut, envs)
+  newψ = changebonds(ψ, alg)
+  return newψ, environments(newψ, H)
 end
 
 changebonds(mpo::FiniteMPOHamiltonian, alg::SvdCut) = changebonds!(copy(mpo), alg)
diff --git a/src/states/finitemps.jl b/src/states/finitemps.jl
index 5bfdcba92..6f65dbbe5 100644
--- a/src/states/finitemps.jl
+++ b/src/states/finitemps.jl
@@ -275,6 +275,15 @@ function FiniteMPS(
     return FiniteMPS(rand, Defaults.eltype, Pspaces, maxVspaces; kwargs...)
 end
 
+
+function FiniteMPS(
+        elt::Type, Pspaces::Vector{<:Union{S, CompositeSpace{S}}}, maxVspaces::Union{S, Vector{S}};
+        kwargs...
+    ) where {S <: ElementarySpace}
+    return FiniteMPS(rand, elt, Pspaces, maxVspaces; kwargs...)
+end
+
+
 # Also accept single physical space and length
 function FiniteMPS(N::Int, V::VectorSpace, args...; kwargs...)
     return FiniteMPS(fill(V, N), args...; kwargs...)

From e1f8e390d9ce3348f52723076a3f2b250772c3f4 Mon Sep 17 00:00:00 2001
From: herviou <loic.herviou@gmail.com>
Date: Fri, 17 Apr 2026 11:27:13 +0200
Subject: [PATCH 2/5] Fixing formatting

---
 src/algorithms/changebonds/randexpand.jl | 4 ++--
 src/algorithms/changebonds/svdcut.jl     | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/algorithms/changebonds/randexpand.jl b/src/algorithms/changebonds/randexpand.jl
index e67a05509..ccf839139 100644
--- a/src/algorithms/changebonds/randexpand.jl
+++ b/src/algorithms/changebonds/randexpand.jl
@@ -51,8 +51,8 @@ changebonds(ψ::AbstractMPS, alg::RandExpand) = changebonds!(copy(ψ), alg)
 changebonds(ψ::MultilineMPS, alg::RandExpand) = changebonds!(copy(ψ), alg)
 
 function changebonds(ψ, H, alg::RandExpand, envs)
-  newψ = changebonds(ψ, alg)    
-  return newψ, environments(newψ, H)
+    newψ = changebonds(ψ, alg)
+    return newψ, environments(newψ, H)
 end
 
 
diff --git a/src/algorithms/changebonds/svdcut.jl b/src/algorithms/changebonds/svdcut.jl
index 2b674d5be..bcdb5b039 100644
--- a/src/algorithms/changebonds/svdcut.jl
+++ b/src/algorithms/changebonds/svdcut.jl
@@ -109,8 +109,8 @@ function changebonds(ψ::InfiniteMPS, alg::SvdCut)
 end
 
 function changebonds(ψ, H, alg::SvdCut, envs)
-  newψ = changebonds(ψ, alg)
-  return newψ, environments(newψ, H)
+    newψ = changebonds(ψ, alg)
+    return newψ, environments(newψ, H)
 end
 
 changebonds(mpo::FiniteMPOHamiltonian, alg::SvdCut) = changebonds!(copy(mpo), alg)

From afdc245f24b30a6ed0823e0fbfd0a08a8d2d0d37 Mon Sep 17 00:00:00 2001
From: herviou <loic.herviou@gmail.com>
Date: Fri, 17 Apr 2026 11:54:55 +0200
Subject: [PATCH 3/5] Fixing an order of calls (no local bug)

---
 src/algorithms/changebonds/changebonds.jl | 13 -------------
 1 file changed, 13 deletions(-)

diff --git a/src/algorithms/changebonds/changebonds.jl b/src/algorithms/changebonds/changebonds.jl
index 3cff7c4b1..009351328 100644
--- a/src/algorithms/changebonds/changebonds.jl
+++ b/src/algorithms/changebonds/changebonds.jl
@@ -9,19 +9,6 @@ See also: [`SvdCut`](@ref), [`RandExpand`](@ref), [`VUMPSSvdCut`](@ref), [`Optim
 function changebonds end
 function changebonds! end
 
-
-##= This is never useful
-# write in terms of MultilineMPS
-function changebonds(
-        ψ::InfiniteMPS, operator::InfiniteMPO, alg::OptimalExpand, envs = environments(ψ, operator)
-    )
-    ψ′, envs′ = changebonds(
-        convert(MultilineMPS, ψ), convert(MultilineMPO, operator), alg, Multiline([envs])
-    )
-    return convert(InfiniteMPS, ψ′), envs
-end
-# =#
-
 _expand(ψ, AL′, AR′) = _expand!(copy(ψ), AL′, AR′)
 function _expand!(ψ::InfiniteMPS, AL′::PeriodicVector, AR′::PeriodicVector)
     for i in 1:length(ψ)

From 9a29b8c9d2211f1d771a90497e7a04ce1212c82f Mon Sep 17 00:00:00 2001
From: herviou <loic.herviou@gmail.com>
Date: Mon, 20 Apr 2026 10:30:21 +0200
Subject: [PATCH 4/5] Lukas proposed change, more explicit calls/type convert

---
 src/algorithms/changebonds/optimalexpand.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/algorithms/changebonds/optimalexpand.jl b/src/algorithms/changebonds/optimalexpand.jl
index 59fd002ca..cefa2dc09 100644
--- a/src/algorithms/changebonds/optimalexpand.jl
+++ b/src/algorithms/changebonds/optimalexpand.jl
@@ -24,7 +24,7 @@ function changebonds(
     ψ′, envs′ = changebonds(
         convert(MultilineMPS, ψ), convert(MultilineMPO, operator), alg, Multiline([envs])
     )
-    return convert(InfiniteMPS, ψ′), envs #This does not sound safe, it relies on the onsite modification of the environments, and the compatibility of the conversion
+    return convert(InfiniteMPS, ψ′), only(parent(envs′))
 end
 # =#
 

From 44d514c31db2367f1e7388c5d71bf064b3ae1918 Mon Sep 17 00:00:00 2001
From: herviou <loic.herviou@gmail.com>
Date: Thu, 30 Apr 2026 10:31:47 +0200
Subject: [PATCH 5/5] Fixing some inconsistencies, and at least adding details
 in docstrings

---
 src/algorithms/changebonds/changebonds.jl   |  3 +++
 src/algorithms/changebonds/optimalexpand.jl |  8 ++++++--
 src/algorithms/changebonds/randexpand.jl    | 10 ++++++++++
 src/algorithms/changebonds/svdcut.jl        |  8 ++++++++
 src/algorithms/changebonds/vumpssvd.jl      |  7 ++++---
 5 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/src/algorithms/changebonds/changebonds.jl b/src/algorithms/changebonds/changebonds.jl
index 009351328..5e7a5bdca 100644
--- a/src/algorithms/changebonds/changebonds.jl
+++ b/src/algorithms/changebonds/changebonds.jl
@@ -3,6 +3,9 @@
     changebonds(ψ::AbstractMPS, alg) -> ψ′
 
 Change the bond dimension of `ψ` using the algorithm `alg`, and return the new `ψ` and the new `envs`.
+For AbstractInfiniteMPS, changebonds returns new environments without modifying the one provided.
+changedbonds! can modifiy both the provided state and environments, depending on the algorithm.
+For FiniteMPS, changebonds also modifies the environments.
 
 See also: [`SvdCut`](@ref), [`RandExpand`](@ref), [`VUMPSSvdCut`](@ref), [`OptimalExpand`](@ref)
 """
diff --git a/src/algorithms/changebonds/optimalexpand.jl b/src/algorithms/changebonds/optimalexpand.jl
index cefa2dc09..5e93a29bd 100644
--- a/src/algorithms/changebonds/optimalexpand.jl
+++ b/src/algorithms/changebonds/optimalexpand.jl
@@ -5,6 +5,8 @@ An algorithm that expands the given mps as described in
 [Zauner-Stauber et al. Phys. Rev. B 97 (2018)](@cite zauner-stauber2018), by selecting the
 dominant contributions of a two-site updated MPS tensor, orthogonal to the original ψ.
 
+changedbonds! is only defined for FiniteMPS, and modify its environment.
+
 ## Fields
 
 $(TYPEDFIELDS)
@@ -18,6 +20,7 @@ $(TYPEDFIELDS)
 end
 
 ##= This is only useful for OptimalExpand
+### Simple wrapper to convert between diffrent type of InifniteMPS.
 function changebonds(
         ψ::InfiniteMPS, operator::InfiniteMPO, alg::OptimalExpand, envs = environments(ψ, operator)
     )
@@ -50,7 +53,7 @@ function changebonds(
     end
 
     newψ = _expand(ψ, AL′, AR′)
-    recalculate!(envs, newψ, H)
+    envs = environments(newψ, H)
     return newψ, envs
 end
 
@@ -75,13 +78,14 @@ function changebonds(ψ::MultilineMPS, H, alg::OptimalExpand, envs = environment
     end
 
     newψ = _expand(ψ, AL′, AR′)
-    recalculate!(envs, newψ, H)
+    envs = environments(newψ, H) #  recalculate!(envs, newψ, H)
     return newψ, envs
 end
 
 function changebonds(ψ::AbstractFiniteMPS, H, alg::OptimalExpand, envs = environments(ψ, H))
     return changebonds!(copy(ψ), H, alg, envs)
 end
+
 function changebonds!(ψ::AbstractFiniteMPS, H, alg::OptimalExpand, envs = environments(ψ, H))
     #inspired by the infinite mps algorithm, alternative is to use https://arxiv.org/pdf/1501.05504.pdf
 
diff --git a/src/algorithms/changebonds/randexpand.jl b/src/algorithms/changebonds/randexpand.jl
index ccf839139..bb1075dd5 100644
--- a/src/algorithms/changebonds/randexpand.jl
+++ b/src/algorithms/changebonds/randexpand.jl
@@ -9,6 +9,9 @@ parallel, and therefore the expansion will never go beyond the local two-site su
 The truncation strategy dictates the number of expanded states, by generating uniformly
 distributed weights for each state in the two-site space and truncating that.
 
+The environments are not used here.
+changebonds! modify both the provided state and environments.
+
 ## Fields
 
 $(TYPEDFIELDS)
@@ -56,6 +59,13 @@ function changebonds(ψ, H, alg::RandExpand, envs)
 end
 
 
+function changebonds!(ψ, H, alg::RandExpand, envs)
+    ψ = changebonds!(ψ, alg)
+    recalculate!(envs, ψ, H)
+    return ψ, envs
+end
+
+
 function changebonds!(ψ::AbstractFiniteMPS, alg::RandExpand)
     for i in 1:(length(ψ) - 1)
         AC2 = randomize!(_transpose_front(ψ.AC[i]) * _transpose_tail(ψ.AR[i + 1]))
diff --git a/src/algorithms/changebonds/svdcut.jl b/src/algorithms/changebonds/svdcut.jl
index bcdb5b039..21b32c52e 100644
--- a/src/algorithms/changebonds/svdcut.jl
+++ b/src/algorithms/changebonds/svdcut.jl
@@ -4,6 +4,8 @@ $(TYPEDEF)
 An algorithm that uses truncated SVD to change the bond dimension of a state or operator.
 This is achieved by a sweeping algorithm that locally performs (optimal) truncations in a gauged basis.
 
+changedbonds! is only defined for FiniteMPS and FiniteMPO.
+
 See also [`changebonds(!)`](@ref changebonds)
 
 ## Fields
@@ -36,6 +38,12 @@ function changebonds!(ψ::AbstractFiniteMPS, alg::SvdCut; normalize::Bool = true
     return normalize ? normalize!(ψ) : ψ
 end
 
+function changebonds!(ψ::AbstractFiniteMPS, H, alg::SvdCut, envs)
+    ψ = changebonds!(ψ, alg)
+    recalculate!(envs, ψ, H)
+    return ψ, envs
+end
+
 # Note: it might be better to go to an MPS representation first
 # such that the SVD cut happens in a canonical form;
 # this would still not be the correct norm, so we will ignore this for now.
diff --git a/src/algorithms/changebonds/vumpssvd.jl b/src/algorithms/changebonds/vumpssvd.jl
index c924976ce..0b59b3af0 100644
--- a/src/algorithms/changebonds/vumpssvd.jl
+++ b/src/algorithms/changebonds/vumpssvd.jl
@@ -3,6 +3,8 @@ $(TYPEDEF)
 
 An algorithm that uses a two-site update step to change the bond dimension of a state.
 
+changedbonds! is not defined.
+
 ## Fields
 
 $(TYPEDFIELDS)
@@ -44,9 +46,8 @@ function changebonds_1(
     collapsed = InfiniteMPS(
         [nstate.AL[1]], nstate.C[1]; alg.alg_gauge.tol, alg.alg_gauge.maxiter
     )
-    recalculate!(envs, collapsed, H, collapsed)
 
-    return collapsed, envs
+    return collapsed, environments(collapsed, H)
 end
 
 function changebonds_n(state::InfiniteMPS, H, alg::VUMPSSvdCut, envs = environments(state, H))
@@ -75,7 +76,7 @@ function changebonds_n(state::InfiniteMPS, H, alg::VUMPSSvdCut, envs = environme
         copied[loc] = AL1
         copied[loc + 1] = AL2
         state = InfiniteMPS(copied; alg.alg_gauge.tol, alg.alg_gauge.maxiter)
-        recalculate!(envs, state, H, state)
+        envs = environments(state, H)
     end
     return state, envs
 end