From 39ff233ddd301d802d746650031c0cad1829ff32 Mon Sep 17 00:00:00 2001
From: leburgel <lander.burgelman@ugent.be>
Date: Mon, 27 Jul 2026 10:55:46 +0200
Subject: [PATCH] Add `fixedpoint_gradient` implementation for
 `OptimKit.FixedPointAlgorithm` solvers

---
 Project.toml                                  |  2 +-
 .../fixed_point_differentiation.jl            | 27 +++++++++++++++++--
 test/gradients/c4v_ctmrg_gradients.jl         |  2 +-
 test/gradients/ctmrg_gradients.jl             |  2 +-
 4 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/Project.toml b/Project.toml
index 68ece85cd..b315d5659 100644
--- a/Project.toml
+++ b/Project.toml
@@ -42,7 +42,7 @@ MPSKit = "0.13.9"
 MPSKitModels = "0.4"
 MatrixAlgebraKit = "0.6.5"
 OhMyThreads = "0.7, 0.8"
-OptimKit = "0.4"
+OptimKit = "0.5"
 Printf = "1"
 Random = "1"
 Statistics = "1"
diff --git a/src/algorithms/optimization/fixed_point_differentiation.jl b/src/algorithms/optimization/fixed_point_differentiation.jl
index 2c2a54991..4b83124b5 100644
--- a/src/algorithms/optimization/fixed_point_differentiation.jl
+++ b/src/algorithms/optimization/fixed_point_differentiation.jl
@@ -93,8 +93,12 @@ end
 FixedPointGradient(; kwargs...) = GradientAlgorithm(; alg = :FixedPointGradient, kwargs...)
 GRADIENT_ALGORITHM_SYMBOLS[:FixedPointGradient] = FixedPointGradient
 
-const FIXEDPOINT_SOLVER_SYMBOLS = IdDict{Symbol, Type{<:Any}}(
-    :GMRES => GMRES, :BiCGStab => BiCGStab, :Arnoldi => Arnoldi,
+const FIXEDPOINT_SOLVER_SYMBOLS = IdDict{Symbol, Any}(
+    :GMRES => GMRES,
+    :BiCGStab => BiCGStab,
+    :Arnoldi => Arnoldi,
+    :AndersonMixing => AndersonMixing,
+    :SimpleIteration => SimpleIteration,
 )
 
 _default_solver_alg(::Type{<:FixedPointGradient}) = Defaults.gradient_fixedpoint_solver_alg
@@ -107,6 +111,15 @@ function _pad_solver_kwargs(::Type{<:Arnoldi}, solver_kwargs)
     )
     return solver_kwargs
 end
+function _pad_solver_kwargs(::Type{<:OptimKit.FixedPointAlgorithm}, solver_kwargs)
+    solver_kwargs = (;
+        gradtol = solver_kwargs.tol, # patch for OptimKit.FixedPointAlgorithm gradient tolerance kwarg name
+        solver_kwargs...,
+    )
+    solver_kwargs = Base.structdiff(solver_kwargs, (; tol = nothing))
+
+    return solver_kwargs
+end
 
 """
 $(TYPEDEF)
@@ -360,3 +373,13 @@ function fixedpoint_gradient(∂E∂x, ∂f∂x, ∂f∂A, x₀, alg::KrylovKit.
 
     return ∂f∂A(y)
 end
+
+function fixedpoint_gradient(x̆, ∂ₓf, ∂ₚf, y₀, alg::OptimKit.FixedPointAlgorithm)
+    fp(y) = x̆ + ∂ₓf(y) # fixed-point condition for geometric sum
+    y, g, = OptimKit.fixedpoint(fp, y₀, alg)
+    if alg.verbosity > 0 && norm(g) > alg.gradtol
+        @warn("gradient fixed-point iteration reached maximal number of iterations without converging: ‖g‖ = $(norm(g))")
+    end
+
+    return ∂ₚf(y)
+end
diff --git a/test/gradients/c4v_ctmrg_gradients.jl b/test/gradients/c4v_ctmrg_gradients.jl
index 2a0c4dab7..1d860adb5 100644
--- a/test/gradients/c4v_ctmrg_gradients.jl
+++ b/test/gradients/c4v_ctmrg_gradients.jl
@@ -25,7 +25,7 @@ ctmrg_algs = [[:C4vCTMRG]]
 projector_algs = [[:C4vEighProjector, :C4vQRProjector]]
 decomposition_rrule_algs = [[:FullPullback, :TruncPullback]]
 gradient_algs = [[nothing, :FixedPointGradient]]
-gradient_solver_algs = [[:GeomSum, :ManualIter, :GMRES, :BiCGStab, :Arnoldi]]
+gradient_solver_algs = [[:GeomSum, :ManualIter, :GMRES, :BiCGStab, :Arnoldi, :AndersonMixing]]
 steps = -0.01:0.005:0.01
 
 # record which rrule alg is compatible with which projector alg
diff --git a/test/gradients/ctmrg_gradients.jl b/test/gradients/ctmrg_gradients.jl
index 8618e139a..b1bd1974b 100644
--- a/test/gradients/ctmrg_gradients.jl
+++ b/test/gradients/ctmrg_gradients.jl
@@ -23,7 +23,7 @@ projector_algs = [[:HalfInfiniteProjector, :FullInfiniteProjector], [:HalfInfini
 svd_rrule_algs = [[:FullPullback, :TruncPullback, :Arnoldi], [:FullPullback, :Arnoldi]]
 gradient_algs = [[nothing, :FixedPointGradient], [:FixedPointGradient]]
 gradient_solver_algs = [
-    [:GeomSum, :ManualIter, :GMRES, :BiCGStab, :Arnoldi],
+    [:GeomSum, :ManualIter, :GMRES, :BiCGStab, :Arnoldi, :AndersonMixing],
     [:GeomSum, :ManualIter, :GMRES, :BiCGStab, :Arnoldi],
 ]
 steps = -0.01:0.005:0.01