From 0c44f67c99f49270e1df6acd7d5557c247f312d1 Mon Sep 17 00:00:00 2001
From: Lukas <37111893+lkdvos@users.noreply.github.com>
Date: Mon, 12 Aug 2024 18:50:06 +0200
Subject: [PATCH] Fix issue where SVDCut produces InfiniteMPS that is not full
 rank (#174)

* Avoid using constructor that expects full-rank matrix in SVDCut algorithm

* Add warning in InfiniteMPS constructor
---
 src/algorithms/changebonds/svdcut.jl | 11 ++++++++++-
 src/states/infinitemps.jl            |  3 +++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/algorithms/changebonds/svdcut.jl b/src/algorithms/changebonds/svdcut.jl
index 95ddff4c5..9ab311c96 100644
--- a/src/algorithms/changebonds/svdcut.jl
+++ b/src/algorithms/changebonds/svdcut.jl
@@ -83,7 +83,16 @@ function changebonds(ψ::InfiniteMPS, alg::SvdCut)
         copied[i + 1] = _transpose_front(U' * _transpose_tail(copied[i + 1]))
     end
 
-    return normalize!(InfiniteMPS(copied, complex(ncr)))
+    # make sure everything is full rank:
+    makefullrank!(copied)
+
+    # if the bond dimension is not changed, we can keep the same center, otherwise recompute
+    ψ = if space(ncr, 1) != space(copied[1], 1)
+        InfiniteMPS(copied)
+    else
+        InfiniteMPS(copied, complex(ncr))
+    end
+    return normalize!(ψ)
 end
 
 function changebonds(ψ, H, alg::SvdCut, envs=environments(ψ, H))
diff --git a/src/states/infinitemps.jl b/src/states/infinitemps.jl
index 93fbd5a24..d1f5d7276 100644
--- a/src/states/infinitemps.jl
+++ b/src/states/infinitemps.jl
@@ -176,6 +176,9 @@ end
 function InfiniteMPS(AL::AbstractVector{<:GenericMPSTensor}, C₀::MPSBondTensor; kwargs...)
     AL = PeriodicArray(copy.(AL))
 
+    all(isfullrank, AL) ||
+        @warn "Constructing an MPS from tensors that are not full rank"
+
     # initialize tensor storage
     AC = similar.(AL)
     AR = similar.(AL)