Fix row and column mismatch in correlation_length with unit cells (#198)

* fix unit cell

* bump version

* add test

* use _next

* do not include cell size in corr length

* Revert "bump version"

This reverts commit 75cf626.

Author: ogauthe

src/algorithms/toolbox.jl

@@ -276,29 +276,25 @@ MPSKit.correlation_length(state, env::CTMRGEnv; num_vals=2, kwargs...) =
     _correlation_length(env; num_vals, kwargs...)
 
 function _correlation_length(env::CTMRGEnv; num_vals=2, kwargs...)
-    T = scalartype(env)
-    ξ_h = Vector{real(T)}(undef, size(env, 2))
-    ξ_v = Vector{real(T)}(undef, size(env, 3))
-    λ_h = Vector{Vector{T}}(undef, size(env, 2))
-    λ_v = Vector{Vector{T}}(undef, size(env, 3))
+    _, n_rows, n_cols = size(env)
 
     # Horizontal
-    λ_h = map(1:size(env, 2)) do r
+    λ_h = map(1:n_rows) do r
         above = InfiniteMPS(env.edges[NORTH, r, :])
-        below = InfiniteMPS(_dag.(env.edges[SOUTH, r, :]))
+        below = InfiniteMPS(_dag.(env.edges[SOUTH, _next(r, n_rows), :]))
         vals = MPSKit.transfer_spectrum(above; below, num_vals, kwargs...)
         return vals ./ abs(vals[1]) # normalize largest eigenvalue
     end
-    ξ_h = map(λ_row -> -1 / log(abs(λ_row[2])), λ_h)
+    ξ_h = map(λ -> -1 / log(abs(λ[2])), λ_h)
 
     # Vertical
-    λ_v = map(1:size(env, 3)) do c
+    λ_v = map(1:n_cols) do c
         above = InfiniteMPS(env.edges[EAST, :, c])
-        below = InfiniteMPS(_dag.(env.edges[WEST, :, c]))
+        below = InfiniteMPS(_dag.(env.edges[WEST, :, _next(c, n_cols)]))
         vals = MPSKit.transfer_spectrum(above; below, num_vals, kwargs...)
         return vals ./ abs(vals[1]) # normalize largest eigenvalue
     end
-    ξ_v = map(λ_row -> -1 / log(abs(λ_row[2])), λ_v)
+    ξ_v = map(λ -> -1 / log(abs(λ[2])), λ_v)
 
     return ξ_h, ξ_v, λ_h, λ_v
 end

test/ctmrg/correlation_length.jl

@@ -0,0 +1,29 @@
+using Test: @test, @testset
+using TensorKit: ←, ⊗, SU2Irrep, Vect, permute, truncdim, truncbelow
+using MPSKit: correlation_length, leading_boundary
+using PEPSKit: CTMRGEnv, InfinitePEPS
+
+@testset "Correlation length" begin
+    # regression test for https://github.com/QuantumKitHub/PEPSKit.jl/issues/197
+    vD = Vect[SU2Irrep](1//2 => 1)
+    vd = Vect[SU2Irrep](2 => 1)
+    tAKLT_A = ones(vd ← vD ⊗ vD ⊗ vD ⊗ vD)
+    tAKLT_B = permute(adjoint(tAKLT_A), (5,), (1, 2, 3, 4))
+
+    ψ = InfinitePEPS([tAKLT_A tAKLT_B; tAKLT_B tAKLT_A])
+    trscheme = truncdim(20) & truncbelow(1e-12)
+    boundary_alg = (; tol=1e-10, trscheme=trscheme, maxiter=1, verbosity=0)
+    env0 = CTMRGEnv(randn, Float64, ψ, oneunit(vD))
+    env, info_ctmrg = leading_boundary(env0, ψ; boundary_alg...)
+
+    ξ_h, ξ_v, λ_h, λ_v = correlation_length(ψ, env)
+    @test ξ_h isa Vector{Float64}
+    @test size(ξ_h) == (2,)
+    @test all(isfinite.(ξ_h))
+    @test all(ξ_h .> 0)
+
+    @test ξ_v isa Vector{Float64}
+    @test size(ξ_v) == (2,)
+    @test all(isfinite.(ξ_v))
+    @test all(ξ_v .> 0)
+end

test/runtests.jl

@@ -32,6 +32,9 @@ end
         @time @safetestset "PEPO" begin
             include("ctmrg/pepo.jl")
         end
+        @time @safetestset "correlation length" begin
+            include("ctmrg/correlation_length.jl")
+        end
     end
     if GROUP == "ALL" || GROUP == "GRADIENTS"
         @time @safetestset "CTMRG gradients" begin