[ITensors] [BUG] random_mps(sites, ψ) with quantum number conservation errors

[christopherdavidwhite2]

Description of bug

I'm trying to make a random MPS with quantum number conservation; random_mps(sites, ψ) results in an error buried pretty far down.

Minimal code demonstrating the bug or unexpected behavior

Minimal runnable code

using ITensors
using Pkg
Pkg.status("ITensors")

Nsites = 2
sites = siteinds("Electron", Nsites; conserve_qns = true)
ψ0 = MPS(sites,["UpDn", "Emp"])
random_mps(sites, ψ0)

Expected output or behavior

I expect a random MPS

Actual output or behavior

Output of minimal runnable code

[9136182c] ITensors v0.6.19
ERROR: LoadError: MethodError: no method matching state(::Index{Vector{Pair{QN, Int64}}}, ::ITensor)

Closest candidates are:
  state(::Index, !Matched::AbstractString; kwargs...)
   @ ITensors ~/.julia/packages/ITensors/FpnkY/src/lib/SiteTypes/src/sitetype.jl:595
  state(::Index, !Matched::Integer)
   @ ITensors ~/.julia/packages/ITensors/FpnkY/src/lib/SiteTypes/src/sitetype.jl:636

Stacktrace:
 [1] (::ITensors.ITensorMPS.var"#398#400"{Vector{Index{Vector{Pair{QN, Int64}}}}, MPS})(j::Int64)
   @ ITensors.ITensorMPS ./essentials.jl:0
 [2] iterate
   @ ./generator.jl:47 [inlined]
 [3] collect
   @ ./array.jl:834 [inlined]
 [4] MPS(eltype::Type{Float64}, sites::Vector{Index{Vector{Pair{QN, Int64}}}}, states_::MPS)
   @ ITensors.ITensorMPS ~/.julia/packages/ITensors/FpnkY/src/lib/ITensorMPS/src/mps.jl:421
 [5] random_mps(rng::Random.TaskLocalRNG, eltype::Type{Float64}, sites::Vector{Index{Vector{Pair{QN, Int64}}}}, state::MPS; linkdims::Int64)
   @ ITensors.ITensorMPS ~/.julia/packages/ITensors/FpnkY/src/lib/ITensorMPS/src/mps.jl:311
 [6] random_mps(rng::Random.TaskLocalRNG, sites::Vector{Index{Vector{Pair{QN, Int64}}}}, state::MPS; linkdims::Int64)
   @ ITensors.ITensorMPS ~/.julia/packages/ITensors/FpnkY/src/lib/ITensorMPS/src/mps.jl:292
 [7] random_mps(sites::Vector{Index{Vector{Pair{QN, Int64}}}}, state::MPS; linkdims::Int64)
   @ ITensors.ITensorMPS ~/.julia/packages/ITensors/FpnkY/src/lib/ITensorMPS/src/mps.jl:283
 [8] random_mps(sites::Vector{Index{Vector{Pair{QN, Int64}}}}, state::MPS)
   @ ITensors.ITensorMPS ~/.julia/packages/ITensors/FpnkY/src/lib/ITensorMPS/src/mps.jl:280
 [9] top-level scope
   @ ~/bugreport.jl:8
in expression starting at /Users/white/bugreport.jl:8

Version information

• Output from versioninfo():

julia> versioninfo()
versioninfo()
Julia Version 1.10.5
Commit 6f3fdf7b362 (2024-08-27 14:19 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin22.4.0)
  CPU: 8 × Apple M3
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, apple-m1)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)

• Output from using Pkg; Pkg.status("ITensors"):

julia> using Pkg; Pkg.status("ITensors")
Status `~/.julia/environments/v1.10/Project.toml`
  [9136182c] ITensors v0.6.19

[mtfishman]

I don't think we ever purposefully supported passing an MPS as the second argument of random_mps (if that works for non-QN tensors, that is most likely just an accident), you should use random_mps(sites, ["UpDn", "Emp"]).

[christopherdavidwhite2]

yep, this works. (Could've sworn (1) I tried this, and (2) the MPS is what I recall from the past, but you'd know better than I do.)

In any case thanks for the help, and sorry for the bother.

[mtfishman]

No worries! Probably we should try to catch that MPS case, if it accidentally works in some cases.