Error when calling `expectation_value`

[sanderdemeyer]

I run into the issue that calling expectation_value for a LocalOperator containing one-site terms, fails. Only when calling it with two-site operators, does it give a result.

Underlying is a minimal breaking example.

using PEPSKit, TensorKit, TensorKitTensors

T = ComplexF64
pspace = ℂ^2
vspace = ℂ^3
envspace = ℂ^10
ctm_alg = SimultaneousCTMRG()

ψ = InfinitePEPS(randn(T, pspace, vspace ⊗ vspace ⊗ vspace' ⊗ vspace'))
env, = leading_boundary(CTMRGEnv(ψ, envspace), ψ, ctm_alg)

(Nr, Nc) = (1, 1)
lattice = InfiniteSquare(Nr, Nc)
spaces = fill(pspace, Nr, Nc)

op_twosite = PEPSKit.LocalOperator(
    spaces,
    (neighbor => SpinOperators.S_x() ⊗ SpinOperators.S_x() for neighbor in nearest_neighbours(lattice))...,
)

op_onesite = PEPSKit.LocalOperator(
    spaces,
    ([idx,] => SpinOperators.S_x() for idx in vertices(lattice))...,
)

op_combination = PEPSKit.LocalOperator(
    spaces,
    (neighbor => SpinOperators.S_x() ⊗ SpinOperators.S_x() for neighbor in nearest_neighbours(lattice))...,
    ([idx,] => SpinOperators.S_x() for idx in vertices(lattice))...,
)

E1 = expectation_value(ψ, op_twosite, env) # this works
E2 = expectation_value(ψ, op_onesite, env) # this does not work
E3 = expectation_value(ψ, op_combination, env) # this does not work

with stacktrace

ERROR: MethodError: no method matching reduced_densitymatrix(::Vector{…}, ::InfinitePEPS{…}, ::InfinitePEPS{…}, ::CTMRGEnv{…})
The function `reduced_densitymatrix` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  reduced_densitymatrix(::Tuple{CartesianIndex{2}, CartesianIndex{2}}, ::InfinitePEPS, ::InfinitePEPS, ::CTMRGEnv)
   @ PEPSKit ~/.julia/packages/PEPSKit/zz9BS/src/algorithms/contractions/localoperator.jl:394
  reduced_densitymatrix(::Tuple{CartesianIndex{2}}, ::InfinitePEPS, ::InfinitePEPS, ::CTMRGEnv)
   @ PEPSKit ~/.julia/packages/PEPSKit/zz9BS/src/algorithms/contractions/localoperator.jl:355
  reduced_densitymatrix(::NTuple{N, CartesianIndex{2}}, ::InfinitePEPS, ::InfinitePEPS, ::CTMRGEnv) where N
   @ PEPSKit ~/.julia/packages/PEPSKit/zz9BS/src/algorithms/contractions/localoperator.jl:312
  ...

Stacktrace:
  [1] (::PEPSKit.var"#560#561"{InfinitePEPS{…}, InfinitePEPS{…}, CTMRGEnv{…}})(::Pair{Any, TensorMap{…} where {…}})
    @ PEPSKit ~/.julia/packages/PEPSKit/zz9BS/src/algorithms/toolbox.jl:16
  [2] iterate
    @ ./generator.jl:48 [inlined]
  [3] collect_to!(dest::Vector{ComplexF64}, itr::Base.Generator{Vector{…}, PEPSKit.var"#560#561"{…}}, offs::Int64, st::Int64)
    @ Base ./array.jl:849
  [4] collect_to_with_first!(dest::Vector{…}, v1::ComplexF64, itr::Base.Generator{…}, st::Int64)
    @ Base ./array.jl:827
  [5] _collect(c::Vector{…}, itr::Base.Generator{…}, ::Base.EltypeUnknown, isz::Base.HasShape{…})
    @ Base ./array.jl:821
  [6] collect_similar(cont::Vector{Pair{…}}, itr::Base.Generator{Vector{…}, PEPSKit.var"#560#561"{…}})
    @ Base ./array.jl:720
  [7] map(f::Function, A::Vector{Pair{Any, TensorMap{ComplexF64, ComplexSpace, N₁, N₂, Vector{ComplexF64}} where {N₁, N₂}}})
    @ Base ./abstractarray.jl:3371
  [8] #tmap#108
    @ ~/.julia/packages/OhMyThreads/qVAdL/src/implementation.jl:453 [inlined]
  [9] dtmap(::Function, ::Vararg{Any}; scheduler::OhMyThreads.Schedulers.SerialScheduler)
    @ PEPSKit ~/.julia/packages/PEPSKit/zz9BS/src/utility/diffable_threads.jl:9
 [10] expectation_value(bra::InfinitePEPS{…}, O::LocalOperator{…}, ket::InfinitePEPS{…}, env::CTMRGEnv{…})
    @ PEPSKit ~/.julia/packages/PEPSKit/zz9BS/src/algorithms/toolbox.jl:15
 [11] expectation_value(peps::InfinitePEPS{…}, O::LocalOperator{…}, env::CTMRGEnv{…})
    @ PEPSKit ~/.julia/packages/PEPSKit/zz9BS/src/algorithms/toolbox.jl:21
 [12] top-level scope
    @ ~/Documents/Doctoraat/ExcitationsKit.jl/test/bugtest.jl:34
Some type information was truncated. Use `show(err)` to see complete types.

[lkdvos]

Thanks for reporting this! Is this using the main branch? (i.e. could you tell me about what versions are used here)

[sanderdemeyer]

I am using the latest version (v0.7.0).

[Yue-Zhengyuan]

For v0.7.0 before the LocalOperator overhaul, you should use (idx,) (a tuple) instead of [idx,] (a vector) to specify the location of terms.

[sanderdemeyer]

@Yue-Zhengyuan this indeed fixes the problem. Thank you!