[BUG] write_when_maxdim_exceeds not working for exci... | ITensor/ITensorMPS.jl

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Description of bug

I seem to be running into a bug regarding "write_when_maxdim_exceeds" when calculating excited states using dmrg. This seems to be a similar bug other people have posted about for other solvers, but I was hoping since "write_when_maxdim_exceeds" works for the ground state dmrg calculation, it may be easily fixed for the subsequent excited state calculations.

Let me know if there is a known work around.

Minimal code demonstrating the bug or unexpected behavior

Minimal runnable code

using ITensors, ITensorMPS

let
  N = 100
  sites = siteinds("S=1",N)

  os = OpSum()
  for j=1:N-1
    os += 0.5,"S+",j,"S-",j+1
    os += 0.5,"S-",j,"S+",j+1
    os += "Sz",j,"Sz",j+1
  end
  H = MPO(os,sites)

  nsweeps = 5 # number of sweeps is 5
  maxdim = [10,20,100,100,200] # gradually increase states kept
  cutoff = [1E-10] # desired truncation error

  psi0 = random_mps(sites;linkdims=2)

  energy,psi = dmrg(H,psi0;nsweeps,maxdim,cutoff, write_when_maxdim_exceeds=1)

  energy2, psi2 = dmrg(H,[psi], psi0;nsweeps,maxdim,cutoff, write_when_maxdim_exceeds=1)

  return
end

Expected output or behavior

I guess I expect the 2nd dmrg calculation done for the first excited state to be able to save to disk.

Actual output or behavior

I get the following error once the 2nd dmrg sweeps start

Output of minimal runnable code

ERROR: LoadError: MethodError: no method matching disk(::ProjMPO_MPS; path::String)
The function `disk` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  disk(::ProjMPO; kwargs...)
   @ ITensorMPS C:\Users\395642\.julia\packages\ITensorMPS\50GxK\src\abstractprojmpo\diskprojmpo.jl:69
  disk(::ITensorMPS.DiskProjMPO; kwargs...)
   @ ITensorMPS C:\Users\395642\.julia\packages\ITensorMPS\50GxK\src\abstractprojmpo\diskprojmpo.jl:82
  disk(::ITensorMPS.AbstractSum; disk_kwargs...)
   @ ITensorMPS C:\Users\395642\.julia\packages\ITensorMPS\50GxK\src\abstractprojmpo\projmposum.jl:118
  ...

Stacktrace:
 [1] dmrg(PH::ProjMPO_MPS, psi0::MPS, sweeps::Sweeps; which_decomp::Nothing, svd_alg::Nothing, observer::NoObserver, outputlevel::Int64, write_when_maxdim_exceeds::Int64, write_path::String, eigsolve_tol::Float64, eigsolve_krylovdim::Int64, eigsolve_maxiter::Int64, eigsolve_verbosity::Int64, eigsolve_which_eigenvalue::Symbol, ishermitian::Bool)
   @ ITensorMPS C:\Users\395642\.julia\packages\ITensorMPS\50GxK\src\dmrg.jl:199
 [2] dmrg
   @ C:\Users\395642\.julia\packages\ITensorMPS\50GxK\src\dmrg.jl:158 [inlined]
 [3] dmrg(H::MPO, Ms::Vector{MPS}, psi0::MPS, sweeps::Sweeps; weight::Bool, kwargs::@Kwargs{outputlevel::Int64, write_when_maxdim_exceeds::Int64})
   @ ITensorMPS C:\Users\395642\.julia\packages\ITensorMPS\50GxK\src\dmrg.jl:55
 [4] dmrg
   @ C:\Users\395642\.julia\packages\ITensorMPS\50GxK\src\dmrg.jl:41 [inlined]
 [5] #dmrg#518
   @ C:\Users\395642\.julia\packages\ITensorMPS\50GxK\src\dmrg.jl:374 [inlined]
 [6] top-level scope
   @ C:\Users\395642\Documents\julia\save_to_disk_test.jl:23
in expression starting at C:\Users\395642\Documents\julia\save_to_disk_test.jl:3

Version information

Output from versioninfo():

julia> versioninfo()
Julia Version 1.11.5
Commit 760b2e5b73 (2025-04-14 06:53 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: 32 × 13th Gen Intel(R) Core(TM) i9-13950HX
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, alderlake)
Threads: 1 default, 0 interactive, 1 GC (on 32 virtual cores)

Output from using Pkg; Pkg.status("ITensorMPS"):

julia> using Pkg; Pkg.status("ITensorMPS")
Status `C:\Users\395642\.julia\environments\v1.11\Project.toml`
  [0d1a4710] ITensorMPS v0.3.18

Reproduce the minimal Julia example, then read src/dmrg.jl around line 199 and the disk methods in src/abstractprojmpo/diskprojmpo.jl. Trace the excited-state dmrg path that receives ProjMPO_MPS and compare it with the working ground-state path. Done means the excited-state calculation can use write_when_maxdim_exceeds without the reported MethodError.

Written by the indexing model from the issue text.

[BUG] write_when_maxdim_exceeds not working for excited state dmrg calculation

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